Micro-optimize reuseDeltaFor in PackWriter

Micro-optimize reuseDeltaFor in PackWriter

This switch is called mostly for OBJ_TREE and OBJ_BLOB types, which
typically make up 66% of the objects in a repository. Simplify the
test for these common types by testing for the one bit they have in
common and returning early.

Object type 5 is currently undefined. In the old code it would hit
the default and return true. In the new code it will match the early
case and also return true. In either implementation 5 should never show
up as it is not a valid type known to Git.

Object type 6 OFS_DELTA is not permitted to be supplied here.
Object type 7 REF_DELTA is not permitted to be supplied here.

Change-Id: I0ede8acee928bb3e73c744450863942064864e9c