Currently, a loose reference is deleted even before locking the
`packed-refs` file, let alone deleting any packed version of the
reference. This leads to two problems, demonstrated by two new tests:
* While a reference is being deleted, other processes might see the
old, packed value of the reference for a moment before the packed
version is deleted. Normally this would be hard to observe, but we
can prolong the window by locking the `packed-refs` file externally
before running `update-ref`, then unlocking it before `update-ref`'s
attempt to acquire the lock times out.
* If the `packed-refs` file is locked so long that `update-ref` fails
to lock it, then the reference can be left permanently in the
incorrect state described in the previous point.
In a moment, both problems will be fixed.
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>