diff --git a/.gitignore b/.gitignore
index e358b18b78..7e77c9d022 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,5 @@
 *~
 git-subtree.xml
 git-subtree.1
+mainline
+subproj
diff --git a/git-subtree.sh b/git-subtree.sh
index 8baa376fe5..66ce251eaa 100755
--- a/git-subtree.sh
+++ b/git-subtree.sh
@@ -16,7 +16,8 @@ git subtree split --prefix=<prefix> <commit...>
 h,help        show the help
 q             quiet
 d             show debug messages
-prefix=       the name of the subdir to split out
+P,prefix=     the name of the subdir to split out
+m,message=    use the given message as the commit message for the merge commit
  options for 'split'
 annotate=     add a prefix to commit message of new commits
 b,branch=     create a new branch from the split subtree
@@ -40,6 +41,7 @@ rejoin=
 ignore_joins=
 annotate=
 squash=
+message=
 
 debug()
 {
@@ -76,7 +78,8 @@ while [ $# -gt 0 ]; do
 		--annotate) annotate="$1"; shift ;;
 		--no-annotate) annotate= ;;
 		-b) branch="$1"; shift ;;
-		--prefix) prefix="$1"; shift ;;
+		-P) prefix="$1"; shift ;;
+		-m) message="$1"; shift ;;
 		--no-prefix) prefix= ;;
 		--onto) onto="$1"; shift ;;
 		--no-onto) onto= ;;
@@ -158,6 +161,20 @@ rev_exists()
 	fi
 }
 
+rev_is_descendant_of_branch()
+{
+	newrev="$1"
+	branch="$2"
+	branch_hash=$(git rev-parse $branch)
+	match=$(git rev-list -1 $branch_hash ^$newrev)
+
+	if [ -z "$match" ]; then
+		return 0
+	else
+		return 1
+	fi
+}
+
 # if a commit doesn't have a parent, this might not work.  But we only want
 # to remove the parent from the rev-list, and since it doesn't exist, it won't
 # be there anyway, so do nothing in that case.
@@ -266,8 +283,13 @@ add_msg()
 	dir="$1"
 	latest_old="$2"
 	latest_new="$3"
+	if [ -n "$message" ]; then
+		commit_message="$message"
+	else
+		commit_message="Add '$dir/' from commit '$latest_new'"
+	fi
 	cat <<-EOF
-		Add '$dir/' from commit '$latest_new'
+		$commit_message
 		
 		git-subtree-dir: $dir
 		git-subtree-mainline: $latest_old
@@ -275,13 +297,27 @@ add_msg()
 	EOF
 }
 
+add_squashed_msg()
+{
+	if [ -n "$message" ]; then
+		echo "$message"
+	else
+		echo "Merge commit '$1' as '$2'"
+	fi
+}
+
 rejoin_msg()
 {
 	dir="$1"
 	latest_old="$2"
 	latest_new="$3"
+	if [ -n "$message" ]; then
+		commit_message="$message"
+	else
+		commit_message="Split '$dir/' into commit '$latest_new'"
+	fi
 	cat <<-EOF
-		Split '$dir/' into commit '$latest_new'
+		$commit_message
 		
 		git-subtree-dir: $dir
 		git-subtree-mainline: $latest_old
@@ -441,7 +477,7 @@ cmd_add()
 	
 	if [ -n "$squash" ]; then
 		rev=$(new_squash_commit "" "" "$rev") || exit $?
-		commit=$(echo "Merge commit '$rev' as '$dir'" |
+		commit=$(add_squashed_msg "$rev" "$dir" |
 			 git commit-tree $tree $headp -p "$rev") || exit $?
 	else
 		commit=$(add_msg "$dir" "$headrev" "$rev" |
@@ -454,10 +490,6 @@ cmd_add()
 
 cmd_split()
 {
-	if [ -n "$branch" ] && rev_exists "refs/heads/$branch"; then
-		die "Branch '$branch' already exists."
-	fi
-
 	debug "Splitting $dir..."
 	cache_setup || exit $?
 	
@@ -488,7 +520,8 @@ cmd_split()
 	eval "$grl" |
 	while read rev parents; do
 		revcount=$(($revcount + 1))
-		say -n "$revcount/$revmax ($createcount)
"
+		say -n "$revcount/$revmax ($createcount)
+"
 		debug "Processing commit: $rev"
 		exists=$(cache_get $rev)
 		if [ -n "$exists" ]; then
@@ -505,7 +538,10 @@ cmd_split()
 		
 		# ugly.  is there no better way to tell if this is a subtree
 		# vs. a mainline commit?  Does it matter?
-		[ -z $tree ] && continue
+		if [ -z $tree ]; then
+			cache_set $rev $rev
+			continue
+		fi
 
 		newrev=$(copy_or_skip "$rev" "$tree" "$newparents") || exit $?
 		debug "  newrev is: $newrev"
@@ -526,9 +562,16 @@ cmd_split()
 			$latest_new >&2 || exit $?
 	fi
 	if [ -n "$branch" ]; then
-		git update-ref -m 'subtree split' "refs/heads/$branch" \
-			$latest_new "" || exit $?
-		say "Created branch '$branch'"
+		if rev_exists "refs/heads/$branch"; then
+			if ! rev_is_descendant_of_branch $latest_new $branch; then
+				die "Branch '$branch' is not an ancestor of commit '$latest_new'."
+			fi
+			action='Updated'
+		else
+			action='Created'
+		fi
+		git update-ref -m 'subtree split' "refs/heads/$branch" $latest_new || exit $?
+		say "$action branch '$branch'"
 	fi
 	echo $latest_new
 	exit 0
@@ -561,7 +604,7 @@ cmd_merge()
 		rev="$new"
 	fi
 	
-	git merge -s subtree $rev
+	git merge -s subtree --message="$message" $rev
 }
 
 cmd_pull()
diff --git a/test.sh b/test.sh
index 12b0456574..cfe3a3c258 100755
--- a/test.sh
+++ b/test.sh
@@ -53,6 +53,16 @@ multiline()
 	done
 }
 
+undo()
+{
+	git reset --hard HEAD~
+}
+
+last_commit_message()
+{
+	git log --format=%s -1
+}
+
 rm -rf mainline subproj
 mkdir mainline subproj
 
@@ -82,7 +92,24 @@ git branch subdir
 
 git fetch ../subproj sub1
 git branch sub1 FETCH_HEAD
+
+# check if --message works for add
+git subtree add --prefix=subdir --message="Added subproject" sub1
+check_equal "$(last_commit_message)" "Added subproject"
+undo
+
+# check if --message works as -m and --prefix as -P
+git subtree add -P subdir -m "Added subproject using git subtree" sub1
+check_equal "$(last_commit_message)" "Added subproject using git subtree"
+undo
+
+# check if --message works with squash too
+git subtree add -P subdir -m "Added subproject with squash" --squash sub1
+check_equal "$(last_commit_message)" "Added subproject with squash"
+undo
+
 git subtree add --prefix=subdir/ FETCH_HEAD
+check_equal "$(last_commit_message)" "Add 'subdir/' from commit '$(git rev-parse sub1)'"
 
 # this shouldn't actually do anything, since FETCH_HEAD is already a parent
 git merge -m 'merge -s -ours' -s ours FETCH_HEAD
@@ -98,13 +125,44 @@ git commit -m 'main-sub7'
 
 git fetch ../subproj sub2
 git branch sub2 FETCH_HEAD
+
+# check if --message works for merge
+git subtree merge --prefix=subdir -m "Merged changes from subproject" sub2
+check_equal "$(last_commit_message)" "Merged changes from subproject"
+undo
+
+# check if --message for merge works with squash too
+git subtree merge --prefix subdir -m "Merged changes from subproject using squash" --squash sub2
+check_equal "$(last_commit_message)" "Merged changes from subproject using squash"
+undo
+
 git subtree merge --prefix=subdir FETCH_HEAD
 git branch pre-split
+check_equal "$(last_commit_message)" "Merge commit '$(git rev-parse sub2)' into mainline"
 
-spl1=$(git subtree split --annotate='*' \
-		--prefix subdir --onto FETCH_HEAD --rejoin)
+# check if --message works for split+rejoin
+spl1=$(git subtree split --annotate='*' --prefix subdir --onto FETCH_HEAD --message "Split & rejoin" --rejoin)
 echo "spl1={$spl1}"
 git branch spl1 "$spl1"
+check_equal "$(last_commit_message)" "Split & rejoin"
+undo
+
+# check split with --branch
+git subtree split --annotate='*' --prefix subdir --onto FETCH_HEAD --branch splitbr1
+check_equal "$(git rev-parse splitbr1)" "$spl1"
+
+# check split with --branch for an existing branch
+git branch splitbr2 sub1
+git subtree split --annotate='*' --prefix subdir --onto FETCH_HEAD --branch splitbr2
+check_equal "$(git rev-parse splitbr2)" "$spl1"
+
+# check split with --branch for an incompatible branch
+result=$(git subtree split --prefix subdir --onto FETCH_HEAD --branch subdir || echo "caught error")
+check_equal "$result" "caught error"
+
+
+git subtree split --annotate='*' --prefix subdir --onto FETCH_HEAD --rejoin
+check_equal "$(last_commit_message)" "Split 'subdir/' into commit '$spl1'"
 
 create subdir/main-sub8
 git commit -m 'main-sub8'
@@ -170,6 +228,50 @@ check_equal "$(git log --pretty=format:'%s' HEAD^2 | grep -i split)" ""
 # meaningless to subproj since one side of the merge refers to the mainline)
 check_equal "$(git log --pretty=format:'%s%n%b' HEAD^2 | grep 'git-subtree.*:')" ""
 
+
+# check if split can find proper base without --onto
+# prepare second pair of repositories
+mkdir test2
+cd test2
+
+mkdir main
+cd main
+git init
+create main1
+git commit -m "main1"
+
+cd ..
+mkdir sub
+cd sub
+git init
+create sub2
+git commit -m "sub2"
+
+cd ../main
+git fetch ../sub master
+git branch sub2 FETCH_HEAD
+git subtree add --prefix subdir sub2
+
+cd ../sub
+create sub3
+git commit -m "sub3"
+
+cd ../main
+git fetch ../sub master
+git branch sub3 FETCH_HEAD
+git subtree merge --prefix subdir sub3
+
+create subdir/main-sub4
+git commit -m "main-sub4"
+git subtree split --prefix subdir --branch mainsub4
+
+# at this point, the new commit's parent should be sub3
+# if it's not, something went wrong (the "newparent" of "master~" commit should have been sub3,
+# but it wasn't, because it's cache was not set to itself)
+check_equal "$(git log --format=%P -1 mainsub4)" "$(git rev-parse sub3)"
+
+
+
 # make sure no patch changes more than one file.  The original set of commits
 # changed only one file each.  A multi-file change would imply that we pruned
 # commits too aggressively.