Speedup GC listing objects referenced from reflogs

Speedup GC listing objects referenced from reflogs

GC needs to get a ReflogReader for all existing refs to list all objects
referenced from reflogs. The existing Repository#getReflogReader method
accepts the ref name and then resolves the Ref to create a ReflogReader.
GC calling that for a huge number of Refs one by one is very slow. GC
first gets all Refs in bulk and then calls getReflogReader for each of
them.

Fix this by adding another getReflogReader method to Repository which
accepts a Ref directly.

This speeds up running JGit gc on a mirror clone of the Gerrit
repository from 15:36 min to 1:08 min. The repository used in this test
had 45k refs, 275k commits and 1.2m git objects.

Change-Id: I474897fdc6652923e35d461c065a29f54d9949f4