Merge branch 'jk/diff-compact-heuristic'

It turns out that the earlier effort to update the heuristics may
want to use a bit more time to mature.  Turn it off by default.

* jk/diff-compact-heuristic:
diff: disable compaction heuristic for now

diff --cc Documentation/diff-config.txt
Simple merge

diff --cc Documentation/diff-options.txt
Simple merge

diff --cc diff.c
Simple merge