Replace unpack_entry_gently with unpack_entry.

Replace unpack_entry_gently with unpack_entry.

The unpack_entry_gently function currently has only two callers:
the delta base resolution in sha1_file.c and the main loop of
pack-check.c.  Both of these must change to using unpack_entry
directly when we implement sliding window mmap logic, so I'm doing
it earlier to help break down the change set.

This may cause a slight performance decrease for delta base
resolution as well as for pack-check.c's verify_packfile(), as
the pack use counter will be incremented and decremented for every
object that is unpacked.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>