git-submodule: add support for --merge.

'git submodule update --merge' merges the commit referenced by the
superproject into your local branch, instead of checking it out on
a detached HEAD.

As evidenced by the addition of "git submodule update --rebase", it
is useful to provide alternatives to the default 'checkout' behaviour
of "git submodule update". One such alternative is, when updating a
submodule to a new commit, to merge that commit into the current
local branch in that submodule. This is useful in workflows where
you want to update your submodule from its upstream, but you cannot
use --rebase, because you have downstream people working on top of
your submodule branch, and you don't want to disrupt their work.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-submodule.txt

@@ -113,8 +113,9 @@ init::
 update::
 	Update the registered submodules, i.e. clone missing submodules and
 	checkout the commit specified in the index of the containing repository.
-	This will make the submodules HEAD be detached unless '--rebase' is
-	specified or the key `submodule.$name.update` is set to	`rebase`.
+	This will make the submodules HEAD be detached unless '--rebase' or
+	'--merge' is specified or the key `submodule.$name.update` is set to
+	`rebase` or `merge`.
 +
 If the submodule is not yet initialized, and you just want to use the
 setting as stored in .gitmodules, you can automatically initialize the
@@ -187,6 +188,16 @@ OPTIONS
 	If the key `submodule.$name.update` is set to `rebase`, this option is
 	implicit.
 
+--merge::
+	This option is only valid for the update command.
+	Merge the commit recorded in the superproject into the current branch
+	of the submodule. If this option is given, the submodule's HEAD will
+	not be detached. If a merge failure prevents this process, you will
+	have to resolve the resulting conflicts within the submodule with the
+	usual conflict resolution tools.
+	If the key `submodule.$name.update` is set to `merge`, this option is
+	implicit.
+
 <path>...::
 	Paths to submodule(s). When specified this will restrict the command
 	to only operate on the submodules found at the specified paths.

Documentation/gitmodules.txt

@@ -35,9 +35,11 @@ submodule.<name>.update::
 	If 'checkout' (the default), the new commit specified in the
 	superproject will be checked out in the submodule on a detached HEAD.
 	If 'rebase', the current branch of the submodule will be rebased onto
-	the commit specified in the superproject.
+	the commit specified in the superproject. If 'merge', the commit
+	specified in the superproject will be merged into the current branch
+	in the submodule.
 	This config option is overridden if 'git submodule update' is given
-	the '--rebase' option.
+	the '--merge' or '--rebase' options.
 
 
 EXAMPLES

git-submodule.sh

@@ -5,7 +5,7 @@
 # Copyright (c) 2007 Lars Hjemli
 
 USAGE="[--quiet] [--cached] \
-[add [-b branch] <repo> <path>]|[status|init|update [-i|--init] [-N|--no-fetch]|summary [-n|--summary-limit <n>] [<commit>]] \
+[add [-b branch] <repo> <path>]|[status|init|update [-i|--init] [-N|--no-fetch] [--rebase|--merge]|summary [-n|--summary-limit <n>] [<commit>]] \
 [--] [<path>...]|[foreach <command>]|[sync [--] [<path>...]]"
 OPTIONS_SPEC=
 . git-sh-setup
@@ -331,6 +331,10 @@ cmd_update()
 			shift
 			update="rebase"
 			;;
+		-m|--merge)
+			shift
+			update="merge"
+			;;
 		--)
 			shift
 			break
@@ -396,6 +400,11 @@ cmd_update()
 				action="rebase"
 				msg="rebased onto"
 				;;
+			merge)
+				command="git merge"
+				action="merge"
+				msg="merged in"
+				;;
 			*)
 				command="git checkout $force -q"
 				action="checkout"

t/t7406-submodule-update.sh

@@ -6,7 +6,7 @@
 test_description='Test updating submodules
 
 This test verifies that "git submodule update" detaches the HEAD of the
-submodule and "git submodule update --rebase" does not detach the HEAD.
+submodule and "git submodule update --rebase/--merge" does not detach the HEAD.
 '
 
 . ./test-lib.sh
@@ -76,6 +76,20 @@ test_expect_success 'submodule update --rebase staying on master' '
 	)
 '
 
+test_expect_success 'submodule update --merge staying on master' '
+	(cd super/submodule &&
+	  git reset --hard HEAD~1
+	) &&
+	(cd super &&
+	 (cd submodule &&
+	  compare_head
+	 ) &&
+	 git submodule update --merge submodule &&
+	 cd submodule &&
+	 compare_head
+	)
+'
+
 test_expect_success 'submodule update - rebase in .git/config' '
 	(cd super &&
 	 git config submodule.submodule.update rebase
@@ -110,6 +124,40 @@ test_expect_success 'submodule update - checkout in .git/config but --rebase giv
 	)
 '
 
+test_expect_success 'submodule update - merge in .git/config' '
+	(cd super &&
+	 git config submodule.submodule.update merge
+	) &&
+	(cd super/submodule &&
+	  git reset --hard HEAD~1
+	) &&
+	(cd super &&
+	 (cd submodule &&
+	  compare_head
+	 ) &&
+	 git submodule update submodule &&
+	 cd submodule &&
+	 compare_head
+	)
+'
+
+test_expect_success 'submodule update - checkout in .git/config but --merge given' '
+	(cd super &&
+	 git config submodule.submodule.update checkout
+	) &&
+	(cd super/submodule &&
+	  git reset --hard HEAD~1
+	) &&
+	(cd super &&
+	 (cd submodule &&
+	  compare_head
+	 ) &&
+	 git submodule update --merge submodule &&
+	 cd submodule &&
+	 compare_head
+	)
+'
+
 test_expect_success 'submodule update - checkout in .git/config' '
 	(cd super &&
 	 git config submodule.submodule.update checkout
@@ -137,4 +185,14 @@ test_expect_success 'submodule init picks up rebase' '
 	)
 '
 
+test_expect_success 'submodule init picks up merge' '
+	(cd super &&
+	 git config submodule.merging.url git://non-existing/git &&
+	 git config submodule.merging.path does-not-matter &&
+	 git config submodule.merging.update merge &&
+	 git submodule init merging &&
+	 test "merge" = $(git config submodule.merging.update)
+	)
+'
+
 test_done