worktree: rename get_worktree_from_repository()

worktree: rename get_worktree_from_repository()

get_worktree_from_repository() returns a struct worktree that
describes the worktree that the repository argument would operate
on. Since 0f779147602 (worktree: remove "the_repository" from
is_current_worktree(), 2026-03-26) that worktree is always the
"current" worktree. Change the name to get_current_worktee() to
reflect better what the function does.

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