Correctly name DeltaBaseCache

Correctly name DeltaBaseCache

This class is used only to cache the unpacked form of an object that
was used as a base for another object.  The theory goes that if an
object is used as a delta base for A, it will probably also be a
delta base for B, C, D, E, etc. and therefore having an unpacked copy
of it on hand will make delta resolution for the others very fast.

However since objects are usually only accessed once, we don't want
to cache everything we unpack, just things that we are likely to
need again.  The only things we need again are the delta bases.
Hence, its a delta base cache.

This gets us the class name UnpackedObjectCache back, so we can
use it to actually create a cache of unpacked object information.

Change-Id: I121f356cf4eca7b80126497264eac22bd5825a1d
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>