SnapshottingRefDirectory: Invalidate snapshot after locking ref for

SnapshottingRefDirectory: Invalidate snapshot after locking ref for
update

When using the SnapshottingRefDirectory, if a thread has already read
packed-refs, then another actor updates packed-refs, the original
thread may create an update that is based on the old cached/snapshotted
packed-refs content. That update could effectively perform a forced
update unintentionally because it is unaware of the new content.

This seems particularly likely to happen in a scenario where a loose
ref was just packed. If the ref was loose, our thread would see the
current ref value (because we don't snapshot loose refs and always read
them from disk), but since there is no loose ref, we expect to find the
current value in packed-refs. However, (before this change) we rely
on our snapshot of packed-refs which does not contain the updated ref
value.

Invalidating the cache after the loose ref is locked ensures that the
ref value does not change again before we read it to perform the update.

Bug: jgit-21
Change-Id: Id10900a99bfd0401a1b9c39d997093af0289649e
Signed-off-by: Nasser Grainawi <quic_nasserg@quicinc.com>