index-pack: fix truncation of off_t in comparison

index-pack: fix truncation of off_t in comparison

Commit c6458e6 (index-pack: kill union delta_base to save
memory, 2015-04-18) refactored the comparison functions used
in sorting and binary searching our delta list. The
resulting code does something like:

  int cmp_offsets(off_t a, off_t b)
  {
	  return a - b;
  }

This works most of the time, but produces nonsensical
results when the difference between the two offsets is
larger than what can be stored in an "int". This can lead to
unresolved deltas if the packsize is larger than 2G (even on
64-bit systems, an int is still typically 32 bits):

  $ git clone git://github.com/mozilla/gecko-dev
  Cloning into 'gecko-dev'...
  remote: Counting objects: 4800161, done.
  remote: Compressing objects: 100% (178/178), done.
  remote: Total 4800161 (delta 88), reused 0 (delta 0), pack-reused 4799978
  Receiving objects: 100% (4800161/4800161), 2.21 GiB | 3.26 MiB/s, done.
  Resolving deltas:  99% (3808820/3811944), completed with 0 local objects.
  fatal: pack has 3124 unresolved deltas
  fatal: index-pack failed

We can fix it by doing direct comparisons between the
offsets and returning constants; the callers only care about
the sign of the comparison, not the magnitude.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>