Give a resize handle for sidebyside diffs.

Give a resize handle for sidebyside diffs.
https://bugs.webkit.org/show_bug.cgi?id=74465

Reviewed by Ryosuke Niwa.

This gives a drag handle so you can make the side-by-side
diff not be 50/50 on a per-file-diff basis. This is useful
for cases like new files where there's no value on the
left side.

* PrettyPatch/PrettyPatch.rb:
* code-review.js:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@102796 268f45cc-cd09-0410-ab3c-d52691b4dbfc