Define LowLevelDiffAlgorithm to bypass re-hashing

When passing to a fallback algorithm, we can avoid creating a new copy
of the hash codes for each sequence by passing in the hashed sequences
directly.  This makes it cheaper to switch from HistogramDiff down to
MyersDiff in a single pass.

Change-Id: Ibf2e81be57c083862eeb134279aed676653bf9b5
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>