worktree: remove "the_repository" from is_current_worktree()

worktree: remove "the_repository" from is_current_worktree()

The "is_current" member of struct worktree was added in 750e8a60d69
(worktree.c: mark current worktree, 2016-04-22) and was used in
8d9fdd7087d (worktree.c: check whether branch is rebased in another
worktree, 2016-04-22) to optionally skip the current worktree when
seeing if a branch is already checked out in die_if_checked_out().

To determine if a worktree is "current" is_current_worktree() compares
the gitdir of the worktree to the gitdir of "the_repository"
and returns true when they match. To get the gitdir of the
worktree it calls get_workree_git_dir() which also depends on
"the_repository". This means that even if "wt->path" matches
"wt->repo->worktree" is_current_worktree(wt) will return false when
"wt->repo" is not "the_repository". Consequently die_if_checked_out()
will fail to skip such a worktree when checking if a branch is already
checked out and may die errounously. Fix this by using the worktree's
repository instance instead of "the_repository" when comparing gitdirs.

The use of "the_repository" in is_current_wortree() comes from
replacing get_git_dir() with repo_get_git_dir() in 246deeac951
(environment: make `get_git_dir()` accept a repository, 2024-09-12). In
get_worktree_git_dir() it comes from replacing git_common_path() with
repo_common_path() in 07242c2a5af (path: drop `git_common_path()`
in favor of `repo_common_path()`, 2025-02-07). In both cases the
replacements appear to have been mechanical.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>