worktree: refactor `repair_worktree_after_gitdir_move()`

worktree: refactor `repair_worktree_after_gitdir_move()`

This refactors `repair_worktree_after_gitdir_move()` to use the new
`write_worktree_linking_files` function. It also preserves the
relativity of the linking files; e.g., if an existing worktree used
absolute paths then the repaired paths will be absolute (and visa-versa).
`repair_worktree_after_gitdir_move()` is used to repair both sets of
worktree linking files if the `.git` directory is moved during a
re-initialization using `git init`.

This also adds a test case for reinitializing a repository that has
relative worktrees.

Signed-off-by: Caleb White <cdwhite3@pm.me>
Signed-off-by: Junio C Hamano <gitster@pobox.com>