[BZ#777231] Add "Per-Hunk Ignore Whitespace" like xxdiff

Submitted by Scott Langley

Description

Could you add this feature like xxdiff has?

http://furius.ca/xxdiff/doc/xxdiff-secrets.html

Per-Hunk Ignore Whitespace

Often in a context of looking at many changes, code reviewers get really annoyed with having to review formatting changes intermixed with code changes. Fortunately, some of these changes can be ignored by diff by ignoring the whitespace characters (and this is also supported by xxdiff) but this does not help for the cases when reformatting involves breaking lines.

In practice, if you're just re-formatting some code, it is often the case that only whitespace differences are appearing in a diff hunk. For this purpose, xxdiff has a special feature that de-emphasizes diff hunks that only differ in whitespace. perhunk.png

The algorithm we use is really simple: given a diff hunk with text on both sides (i.e. a change hunk), we compare all the non-whitespace characters. If the entire hunks do not differ in that way, we de-emphasize the hunk to make it look like both regions are the same (using a gray color similar to the color for non-differing hunks). Those hunks are otherwise still selectable for merging.