worktree: add details to the worktree struct

worktree: add details to the worktree struct

In addition to the absolute path in the worktree struct, add the location
of the git dir, the head ref (if not detached), the head revision sha1,
whether or not head is detached, and whether or not the worktree is a
bare repo.

Signed-off-by: Michael Rappazzo <rappazzo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>