Merge branch 'cw/worktree-relative'

An extra worktree attached to a repository points at each other to
allow finding the repository from the worktree and vice versa
possible.  Turn this linkage to relative paths.

* cw/worktree-relative:
  worktree: add test for path handling in linked worktrees
  worktree: link worktrees with relative paths
  worktree: refactor infer_backlink() to use *strbuf
  worktree: repair copied repository and linked worktrees