worktree: add skeleton "repair" command

worktree: add skeleton "repair" command

Worktree administrative files can become corrupted or outdated due to
external factors. Although, it is often possible to recover from such
situations by hand-tweaking these files, doing so requires intimate
knowledge of worktree internals. While information necessary to make
such repairs manually can be obtained from git-worktree.txt and
gitrepository-layout.txt, we can assist users more directly by teaching
git-worktree how to repair its administrative files itself (at least to
some extent). Therefore, add a "git worktree repair" command which
attempts to correct common problems which may arise due to factors
beyond Git's control.

At this stage, the "repair" command is a mere skeleton; subsequent
commits will flesh out the functionality.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>