[RFC PATCH, WAS: "weird diff output?" v3a 0/2] implement shortest line diff chunk heuristic

This is a version based on Jacobs v2, with the same fixes as in his v3 (hopefully), changing the heuristic, such that CRLF confusion might be gone.

TODO:
* add some tests
* think about whether we need a git attribute or not (I did some
  thinking, and if we do need to configure this at all, this is where I
  would put it)
  
Later on we want to have git attributes I'd think. For now let's just keep
the `git config diff.shortestlineheuristic true` config option for testing?
  
Changes since Jacobs v2:
 * s/empty line/shortest line/g
   That new heuristic is a superset of the empty line heuristic as empty lines
   are shortest lines.  This solves the "What is an empty line?" question
   (Think of CRLF vs LF)
 * fixed Jacobs rebase mistake (which is also fixed in Jacobs v3)

Changes since my v1:
* rename xdl_hash_and_recmatch to recs_match
* remove counting empty lines in the first section of the looping

Changes since Stefan's v1:
* Added a patch to implement xdl_hash_and_recmatch as Junio suggested.
* Fixed a segfault in Stefan's patch
* Added XDL flag to configure the behavior
* Used an int and counted empty lines via += instead of |=
* Renamed starts_with_emptyline to is_emptyline
* Added diff command line and config options

Jacob Keller (1):
  xdiff: add recs_match helper function

Stefan Beller (1):
  xdiff: implement empty line chunk heuristic

 Documentation/diff-config.txt  |  6 ++++++
 Documentation/diff-options.txt |  6 ++++++
 diff.c                         | 11 +++++++++++
 xdiff/xdiff.h                  |  2 ++
 xdiff/xdiffi.c                 | 43 ++++++++++++++++++++++++++++++++++++++----
 5 files changed, 64 insertions(+), 4 deletions(-)