rebase-merges: try and use branch names as labels

When interactively rebasing merge commits, the commit message is parsed to
extract a probably meaningful label name. For instance if the merge commit
is “Merge branch 'feature0'”, then the rebase script will have thes lines:
```
label feature0

merge -C $sha feature0 # “Merge branch 'feature0'
```

This heuristic fails in the case of octopus merges or when the merge commit
message is actually unrelated to the parent commits.

An example that combines both is:
```
*---.   967bfa4 (HEAD -> integration) Integration
|\ \ \
| | | * 2135be1 (feature2, feat2) Feature 2
| |_|/
|/| |
| | * c88b01a Feature 1
| |/
|/|
| * 75f3139 (feat0) Feature 0
|/
* 25c86d0 (main) Initial commit
```
yields the labels Integration, Integration-2 and Integration-3.

Fix this by using a branch name for each merge commit's parent that is the
tip of at least one branch, and falling back to a label derived from the
merge commit message otherwise.
In the example above, the labels become feat0, Integration and feature2.

Signed-off-by: Nicolas Guichard <nicolas@guichard.eu>
Acked-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -5819,7 +5819,7 @@ static int make_script_with_merges(struct pretty_print_context *pp,
 	int root_with_onto = flags & TODO_LIST_ROOT_WITH_ONTO;
 	int skipped_commit = 0;
 	struct strbuf buf = STRBUF_INIT, oneline = STRBUF_INIT;
-	struct strbuf label = STRBUF_INIT;
+	struct strbuf label_from_message = STRBUF_INIT;
 	struct commit_list *commits = NULL, **tail = &commits, *iter;
 	struct commit_list *tips = NULL, **tips_tail = &tips;
 	struct commit *commit;
@@ -5842,6 +5842,7 @@ static int make_script_with_merges(struct pretty_print_context *pp,
 	oidmap_init(&state.commit2label, 0);
 	hashmap_init(&state.labels, labels_cmp, NULL, 0);
 	strbuf_init(&state.buf, 32);
+	load_branch_decorations();
 
 	if (revs->cmdline.nr && (revs->cmdline.rev[0].flags & BOTTOM)) {
 		struct labels_entry *onto_label_entry;
@@ -5902,18 +5903,18 @@ static int make_script_with_merges(struct pretty_print_context *pp,
 			continue;
 		}
 
-		/* Create a label */
-		strbuf_reset(&label);
+		/* Create a label from the commit message */
+		strbuf_reset(&label_from_message);
 		if (skip_prefix(oneline.buf, "Merge ", &p1) &&
 		    (p1 = strchr(p1, '\'')) &&
 		    (p2 = strchr(++p1, '\'')))
-			strbuf_add(&label, p1, p2 - p1);
+			strbuf_add(&label_from_message, p1, p2 - p1);
 		else if (skip_prefix(oneline.buf, "Merge pull request ",
 				     &p1) &&
 			 (p1 = strstr(p1, " from ")))
-			strbuf_addstr(&label, p1 + strlen(" from "));
+			strbuf_addstr(&label_from_message, p1 + strlen(" from "));
 		else
-			strbuf_addbuf(&label, &oneline);
+			strbuf_addbuf(&label_from_message, &oneline);
 
 		strbuf_reset(&buf);
 		strbuf_addf(&buf, "%s -C %s",
@@ -5921,6 +5922,14 @@ static int make_script_with_merges(struct pretty_print_context *pp,
 
 		/* label the tips of merged branches */
 		for (; to_merge; to_merge = to_merge->next) {
+			const char *label = label_from_message.buf;
+			const struct name_decoration *decoration =
+				get_name_decoration(&to_merge->item->object);
+
+			if (decoration)
+				skip_prefix(decoration->name, "refs/heads/",
+					    &label);
+
 			oid = &to_merge->item->object.oid;
 			strbuf_addch(&buf, ' ');
 
@@ -5933,7 +5942,7 @@ static int make_script_with_merges(struct pretty_print_context *pp,
 			tips_tail = &commit_list_insert(to_merge->item,
 							tips_tail)->next;
 
-			strbuf_addstr(&buf, label_oid(oid, label.buf, &state));
+			strbuf_addstr(&buf, label_oid(oid, label, &state));
 		}
 		strbuf_addf(&buf, " # %s", oneline.buf);
 
@@ -6041,7 +6050,7 @@ static int make_script_with_merges(struct pretty_print_context *pp,
 	free_commit_list(commits);
 	free_commit_list(tips);
 
-	strbuf_release(&label);
+	strbuf_release(&label_from_message);
 	strbuf_release(&oneline);
 	strbuf_release(&buf);
 

t/t3404-rebase-interactive.sh

@@ -1870,7 +1870,7 @@ test_expect_success '--update-refs adds commands with --rebase-merges' '
 		pick $(git log -1 --format=%h branch2~1) F
 		pick $(git log -1 --format=%h branch2) I
 		update-ref refs/heads/branch2
-		label merge
+		label branch2
 		reset onto
 		pick $(git log -1 --format=%h refs/heads/second) J
 		update-ref refs/heads/second
@@ -1881,7 +1881,7 @@ test_expect_success '--update-refs adds commands with --rebase-merges' '
 		update-ref refs/heads/third
 		pick $(git log -1 --format=%h HEAD~2) M
 		update-ref refs/heads/no-conflict-branch
-		merge -C $(git log -1 --format=%h HEAD~1) merge # merge
+		merge -C $(git log -1 --format=%h HEAD~1) branch2 # merge
 		update-ref refs/heads/merge-branch
 		EOF
 

t/t3430-rebase-merges.sh

@@ -108,19 +108,19 @@ test_expect_success 'generate correct todo list' '
 
 	reset onto
 	pick $b B
-	label E
+	label first
 
 	reset onto
 	pick $c C
 	label branch-point
 	pick $f F
 	pick $g G
-	label H
+	label second
 
 	reset branch-point # C
 	pick $d D
-	merge -C $e E # E
-	merge -C $h H # H
+	merge -C $e first # E
+	merge -C $h second # H
 
 	EOF
 
@@ -462,11 +462,11 @@ test_expect_success 'A root commit can be a cousin, treat it that way' '
 '
 
 test_expect_success 'labels that are object IDs are rewritten' '
-	git checkout -b third B &&
+	git checkout --detach B &&
 	test_commit I &&
 	third=$(git rev-parse HEAD) &&
 	git checkout -b labels main &&
-	git merge --no-commit third &&
+	git merge --no-commit $third &&
 	test_tick &&
 	git commit -m "Merge commit '\''$third'\'' into labels" &&
 	echo noop >script-from-scratch &&