AU: bsdiff: Expand pathological case where files differ by <8 bytes

Modify bsdiff to better handle the case where files differ by <8 bytes
in some regions, not limitting this case to linear traversal.

BUG=chromium-os:28552
TEST=Manual bsdiff of problematic files, update engine unit tests

Change-Id: I5b796977fc0e81d08b2e22abae48ae9e7dd2dba6
Reviewed-on: https://gerrit.chromium.org/gerrit/19270


Reviewed-by: Don Garrett <dgarrett@chromium.org>
Commit-Ready: Thieu Le <thieule@chromium.org>
Tested-by: Thieu Le <thieule@chromium.org>