Merge branch 'pw/worktree-reduce-the-repository'

Reduce the reference to the_repository in the worktree subsystem.

* pw/worktree-reduce-the-repository:
  worktree: reject NULL worktree in get_worktree_git_dir()
  worktree add: stop reading ".git/HEAD"
  worktree: remove "the_repository" from is_current_worktree()

diff --cc builtin/worktree.c
Simple merge

diff --cc worktree.c
Simple merge

diff --cc worktree.h
Simple merge