Don't search for intra-line diffs in really long lines

Don't search for intra-line diffs in really long lines

Doing so can lead to hangs (or at least really slow execution).

Fixes <http://webkit.org/b/56109> run-webkit-tests sometimes times out on Windows XP Debug
(Tests) after fast/text/large-text-composed-char-dos.html fails (due to PrettyPatch
hanging?)

Reviewed by David Kilzer.

* PrettyPatch/PrettyPatch.rb:
(PrettyPatch.MAXIMUM_INTRALINE_DIFF_LINE_LENGTH): Added this new constant.
(PrettyPatch.DiffSection.initialize): Don't bother looking for intra-line diffs in lines
longer than the maximum length.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@80741 268f45cc-cd09-0410-ab3c-d52691b4dbfc