commit: do not lose SQUASH_MSG contents

When concluding a conflicted "git merge --squash", the command
failed to read SQUASH_MSG that was prepared by "git merge", and
showed only the "# Conflicts:" list of conflicted paths.

Place the contents from SQUASH_MSG at the beginning, just like we
show the commit log skeleton first when concluding a normal merge,
and then show the "# Conflicts:" list, to help the user write the
log message for the resulting commit.

Test by Junio C Hamano <gitster@pobox.com>.

Signed-off-by: Sven Strickroth <sven@cs-ware.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit.c

@@ -726,9 +726,18 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 				      &sb, &ctx);
 		hook_arg1 = "message";
 	} else if (!stat(git_path_merge_msg(), &statbuf)) {
+		/*
+		 * prepend SQUASH_MSG here if it exists and a
+		 * "merge --squash" was originally performed
+		 */
+		if (!stat(git_path_squash_msg(), &statbuf)) {
+			if (strbuf_read_file(&sb, git_path_squash_msg(), 0) < 0)
+				die_errno(_("could not read SQUASH_MSG"));
+			hook_arg1 = "squash";
+		} else
+			hook_arg1 = "merge";
 		if (strbuf_read_file(&sb, git_path_merge_msg(), 0) < 0)
 			die_errno(_("could not read MERGE_MSG"));
-		hook_arg1 = "merge";
 	} else if (!stat(git_path_squash_msg(), &statbuf)) {
 		if (strbuf_read_file(&sb, git_path_squash_msg(), 0) < 0)
 			die_errno(_("could not read SQUASH_MSG"));

t/t7600-merge.sh

@@ -33,9 +33,11 @@ printf '%s\n' 1 2 3 4 5 6 7 8 9 >file
 printf '%s\n' '1 X' 2 3 4 5 6 7 8 9 >file.1
 printf '%s\n' 1 2 3 4 '5 X' 6 7 8 9 >file.5
 printf '%s\n' 1 2 3 4 5 6 7 8 '9 X' >file.9
+printf '%s\n' 1 2 3 4 5 6 7 8 '9 Y' >file.9y
 printf '%s\n' '1 X' 2 3 4 5 6 7 8 9 >result.1
 printf '%s\n' '1 X' 2 3 4 '5 X' 6 7 8 9 >result.1-5
 printf '%s\n' '1 X' 2 3 4 '5 X' 6 7 8 '9 X' >result.1-5-9
+printf '%s\n' 1 2 3 4 5 6 7 8 '9 Z' >result.9z
 >empty
 
 create_merge_msgs () {
@@ -128,6 +130,12 @@ test_expect_success 'setup' '
 	git tag c2 &&
 	c2=$(git rev-parse HEAD) &&
 	git reset --hard "$c0" &&
+	cp file.9y file &&
+	git add file &&
+	test_tick &&
+	git commit -m "commit 7" &&
+	git tag c7 &&
+	git reset --hard "$c0" &&
 	cp file.9 file &&
 	git add file &&
 	test_tick &&
@@ -218,6 +226,26 @@ test_expect_success 'merge c1 with c2' '
 	verify_parents $c1 $c2
 '
 
+test_expect_success 'merge --squash c3 with c7' '
+	git reset --hard c3 &&
+	test_must_fail git merge --squash c7 &&
+	cat result.9z >file &&
+	git commit --no-edit -a &&
+
+	{
+		cat <<-EOF
+		Squashed commit of the following:
+
+		$(git show -s c7)
+
+		# Conflicts:
+		#	file
+		EOF
+	} >expect &&
+	git cat-file commit HEAD | sed -e '1,/^$/d' >actual &&
+	test_cmp expect actual
+'
+
 test_debug 'git log --graph --decorate --oneline --all'
 
 test_expect_success 'merge c1 with c2 and c3' '