watchlist: Add support for matching added or deleted lines.

watchlist: Add support for matching added or deleted lines.
https://bugs.webkit.org/show_bug.cgi?id=68972

Reviewed by Adam Barth.

* Scripts/webkitpy/common/watchlist/changedlinepattern.py: Added.
* Scripts/webkitpy/common/watchlist/changedlinepattern_unittest.py: Added.
* Scripts/webkitpy/common/watchlist/watchlist.py: Comment fix up and fix input
to the pattern match to only have the diff lines instead of the DiffFile.
* Scripts/webkitpy/common/watchlist/watchlist_unittest.py:
Add tests for the new patterns and combinations of the patterns.
* Scripts/webkitpy/common/watchlist/watchlistparser.py:
Sort imports. Add changeline support.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@96228 268f45cc-cd09-0410-ab3c-d52691b4dbfc