dotgit: handle refs that exist in both packed-refs and a loose ref file

[strib]

If a repo starts out with a packed-refs file, and later a reference is updated by creating a new reference file for a name that already exists in packed-refs, then the same reference will be listed in two places (possibly with two different ref values). This is legal in git; the loose ref file always takes precedence over the packed-ref file. However, to support this properly, go-git needs to do two things:

1. On ref removal, remove it from both places. First remove the ref file, and then remove the packed-refs file (under a lock). Note that if the underlying storage layer is distributed and might be accessed concurrently, we'd likely require stronger locking semantics to make sure the (possibly out-of-date) packed-refs value isn't accessed after the loose ref is deleted but before packed-refs is cleaned up, but I leave that for future work.
2. When listing refs, don't double-report refs in packed-refs. This used to be the case, but recently (dotgit: remove ref cache for packed refs #651) it was changed, so this restores the original logic (without adding the packed-refs cache back in).

[strib]

(If #665 is merged first, we'll need to update the test in this PR to get it to compile, so please don't blindly merge it.)

[mcuadros]

@strib merge yourself when you are ready.