Merge branch 'nd/corrupt-worktrees' into maint

"git worktree add" used to fail when another worktree connected to
the same repository was corrupt, which has been corrected.

* nd/corrupt-worktrees:
  worktree add: be tolerant of corrupt worktrees

diff --cc t/t2400-worktree-add.sh
Simple merge

diff --cc worktree.c
Simple merge