Invalidate DfsObjDatabase pack list when refs are updated

Invalidate DfsObjDatabase pack list when refs are updated

Currently, there is a race where a user of a DfsRepository in a single
thread may get unexpected MissingObjectExceptions trying to look up an
object that appears as the current value of a ref:

1. Thread A scans packs before scanning refs, for example by reading
   an object by SHA-1.
2. Thread B flushes an object and updates a ref to point to that
   object.
3. Thread A looks up the ref updated in (2). Since it is scanning refs
   for the first time, it sees the new object SHA-1.
4. Thread A tries to read the object it found in (3), using the cached
   pack list it got from (1). The object appears missing.

Allow implementations to work around this by marking the object
database's current pack list as "dirty." A dirty pack list means that
DfsReader will rescan packs and try again if a requested object is
missing. Implementations should mark objects as dirty any time the ref
database reads or scans refs that might be newer than a previously
cached pack list.

Change-Id: I06c722b20c859ed1475628ec6a2f6d3d6d580700