diff: disable compaction heuristic for now

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>

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;