We already have these objects parsed and cached in our object pool.
We shouldn't be looking them up via the legacy mapObject API, but
instead can use the pool and the faster parsing routines available
through the RevWalk that we extend.
While we are here fixing the code, lets also correct the tag date
sorting to accept tags that have no tagger identity, because they
were created before Git knew how to store that field.
Change-Id: Id49a11f6d9c050c82b876e5e11058840c894b2d7
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
599c0ce745