rev-parse: take prefix into account in --git-common-dir

Most of the time, get_git_common_dir() returns an absolute path so
prefix is irrelevant. If it returns a relative path (e.g. from the
main worktree) then prefixing is required.

Noticed-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rev-parse.c

@@ -763,7 +763,8 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 				continue;
 			}
 			if (!strcmp(arg, "--git-common-dir")) {
-				puts(get_git_common_dir());
+				const char *pfx = prefix ? prefix : "";
+				puts(prefix_filename(pfx, strlen(pfx), get_git_common_dir()));
 				continue;
 			}
 			if (!strcmp(arg, "--resolve-git-dir")) {

t/t2027-worktree-list.sh

@@ -8,6 +8,16 @@ test_expect_success 'setup' '
 	test_commit init
 '
 
+test_expect_success 'rev-parse --git-common-dir on main worktree' '
+	git rev-parse --git-common-dir >actual &&
+	echo .git >expected &&
+	test_cmp expected actual &&
+	mkdir sub &&
+	git -C sub rev-parse --git-common-dir >actual2 &&
+	echo sub/.git >expected2 &&
+	test_cmp expected2 actual2
+'
+
 test_expect_success '"list" all worktrees from main' '
 	echo "$(git rev-parse --show-toplevel) $(git rev-parse --short HEAD) [$(git symbolic-ref --short HEAD)]" >expect &&
 	test_when_finished "rm -rf here && git worktree prune" &&