merge-file: fix BUG when --object-id is used in a worktree

The `--object-id` option was added in commit e1068f0
(merge-file: add an option to process object IDs, 2023-11-01)
together with a call to setup_git_directory() to avoid crashing
when run outside a repository.

However, the call to setup_git_directory() is redundant when run inside
a repository, as merge-file runs with RUN_SETUP_GENTLY, so the
repository has already been set up. The redundant call is harmless
when linked worktrees are not used, but in a linked worktree,
the repo_set_gitdir() function ends up being called twice.

Calling repo_set_gitdir() used to be silently accepted, but commit
2816b74 (odb: handle changing a repository's commondir, 2025-11-19)
changed this to a BUG in repository.c with the error message:
"cannot reinitialize an already-initialized object directory".

Guard the redundant call to setup_git_directory() behind a repo pointer
check, to ensure that we continue to give the correct "not a git repo"
error whilst avoiding the BUG when running in a linked worktree.

Signed-off-by: Mathias Rav <m@git.strova.dk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Mortal

builtin/merge-file.c

@@ -60,7 +60,7 @@ static int diff_algorithm_cb(const struct option *opt,
 int cmd_merge_file(int argc,
 		   const char **argv,
 		   const char *prefix,
-		   struct repository *repo UNUSED)
+		   struct repository *repo)
 {
 	const char *names[3] = { 0 };
 	mmfile_t mmfs[3] = { 0 };
@@ -110,7 +110,8 @@ int cmd_merge_file(int argc,
 			return error_errno("failed to redirect stderr to /dev/null");
 	}
 
-	if (object_id)
+	if (!repo && object_id)
+		/* emit the correct "not a git repo" error in this case */
 		setup_git_directory();
 
 	for (i = 0; i < 3; i++) {

t/t6403-merge-file.sh

@@ -506,6 +506,15 @@ test_expect_success '--object-id fails without repository' '
 	grep "not a git repository" err
 '
 
+test_expect_success 'run in a linked worktree with --object-id' '
+	empty="$(test_oid empty_blob)" &&
+	git worktree add work &&
+	git -C work merge-file --object-id $empty $empty $empty >actual &&
+	git worktree remove work &&
+	git merge-file --object-id $empty $empty $empty >expected &&
+	test_cmp actual expected
+'
+
 test_expect_success 'merging C files with "myers" diff algorithm creates some spurious conflicts' '
 	cat >expect.c <<-\EOF &&
 	int g(size_t u)