Test suite for sliding window mmap implementation.

Test suite for sliding window mmap implementation.

This is a basic set of tests for the sliding window mmap.  We mostly
focus on the verify-pack and pack-objects implementations (including
delta reuse) as these commands appear to cover the bulk of the
affected portions of sha1_file.c.

The test cases don't verify the virtual memory size used, as
this can differ from system to system.  Instead it just verifies
that we can run with very low values for core.packedGitLimit and
core.packedGitWindowSize.

Adding pack_report() to the end of both builtin-verify-pack.c and
builtin-pack-objects.c and manually inspecting the statistics output
can help to verify that the total virtual memory size attributed
to pack mmap usage is what one might expect on the current system.

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