Use HistogramDiff by default in DiffFormatter

Its behavior is similar to PatienceDiff, and runs nearly as fast,
often beating the performance of MyersDiff.

Change-Id: I43c3faefa8109f1a68ef57522bec9cf27b5df252
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
1 file changed