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

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

The `--object-id` option was added in commit e1068f0ad4
(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
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 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>