diff: disable compaction heuristic for nowjk/diff-compact-heuristic

http://lkml.kernel.org/g/20160610075043.GA13411@sigill.intra.peff.net
reports that a change to add a new "function" with common ending
with the existing one at the end of the file is shown like this:

    def foo
      do_foo_stuff()

   +  common_ending()
   +end
   +
   +def bar
   +  do_bar_stuff()
   +
      common_ending()
    end

when the new heuristic is in use.  In reality, the change is to add
the blank line before "def bar" and everything below, which is what
the code without the new heuristic shows.

Disable the heuristics by default, and resurrect the documentation
for the option and the configuration variables, while clearly
marking the feature as still experimental.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

3 files changed, 13 insertions, 1 deletions

diff --git a/Documentation/diff-config.txt b/Documentation/diff-config.txt
index 6eaa45271c..6fb70c5d43 100644
--- a/Documentation/diff-config.txt
+++ b/Documentation/diff-config.txt
@@ -166,6 +166,11 @@ diff.tool::
 
 include::mergetools-diff.txt[]
 
+diff.compactionHeuristic::
+	Set this option to `true` to enable an experimental heuristic that
+	shifts the hunk boundary in an attempt to make the resulting
+	patch easier to read.
+
 diff.algorithm::
 	Choose a diff algorithm.  The variants are as follows:
 +
diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
index 3ad6404dbc..9baf1ad277 100644
--- a/Documentation/diff-options.txt
+++ b/Documentation/diff-options.txt
@@ -63,6 +63,13 @@ ifndef::git-format-patch[]
 	Synonym for `-p --raw`.
 endif::git-format-patch[]
 
+--compaction-heuristic::
+--no-compaction-heuristic::
+	These are to help debugging and tuning an experimental
+	heuristic (which is off by default) that shifts the hunk
+	boundary in an attempt to make the resulting patch easier
+	to read.
+
 --minimal::
 	Spend extra time to make sure the smallest possible
 	diff is produced.
diff --git a/diff.c b/diff.c
index 05ca3ce08e..9116d9d1c7 100644
--- a/diff.c
+++ b/diff.c
@@ -25,7 +25,7 @@
 #endif
 
 static int diff_detect_rename_default;
-static int diff_compaction_heuristic = 1;
+static int diff_compaction_heuristic; /* experimental */
 static int diff_rename_limit_default = 400;
 static int diff_suppress_blank_empty;
 static int diff_use_color_default = -1;