The function `git_repository_head_for_worktree` currently uses
`git_reference__read_head` to directly read a given worktree's HEAD from
the filesystem. This is broken in case the repository uses a different
refdb implementation than the filesystem-based one, so let's instead
open the worktree as a real repository and use `git_reference_lookup`.
This also fixes the case where the worktree's HEAD is not a symref, but
a detached HEAD, which would have resulted in an error previously.