> 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
> worktrees are not used, but when run inside a worktree, the
> repo_set_gitdir() function ends up being called twice.
>
> Calling repo_set_gitdir() used to be silently accepted, but commit
> 2816b748e5 ("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 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 inside a worktree.
>
> Signed-off-by: Mathias Rav <m@git.strova.dk>
> ---
>  builtin/merge-file.c  | 4 ++--
>  t/t6403-merge-file.sh | 9 +++++++++
>  2 files changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/builtin/merge-file.c b/builtin/merge-file.c
> index 46775d0c79..a8768c6e0c 100644
> --- a/builtin/merge-file.c
> +++ b/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 +110,7 @@ int cmd_merge_file(int argc,
>  			return error_errno("failed to redirect stderr to /dev/null");
>  	}
>
> -	if (object_id)
> +	if (object_id && !repo)
>  		setup_git_directory();
>
>  	for (i = 0; i < 3; i++) {
> diff --git a/t/t6403-merge-file.sh b/t/t6403-merge-file.sh
> index 06ab4d7aed..60cc43775f 100755
> --- a/t/t6403-merge-file.sh
> +++ b/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 inside worktree with --object-id' '
> +	empty="$(test_oid empty_blob)" &&
> +	git worktree add work &&
> +	(cd work && git 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)
> --
> 2.53.0