Convert unpack_entry_gently and friends to use offsets.

Convert unpack_entry_gently and friends to use offsets.

Change unpack_entry_gently and its helper functions to use offsets
rather than addresses and left counts to supply pack position
information.  In most cases this makes the code easier to follow,
and it reduces the number of local variables in a few functions.
It also better prepares this code for mapping partial segments of
packs and altering what regions of a pack are mapped while unpacking
an entry.

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