prepare-Changelog should support updating the list of changed files
https://bugs.webkit.org/show_bug.cgi?id=74358

Patch by Timothy Loh <timloh@chromium.com> on 2013-01-22
Reviewed by Eric Seidel.

Needing to re-make ChangeLog entries when the list of files/functions
changes is a bit annoying, it'd be nice to have this more automated.
This patch makes `webkit-patch update' update the date line and bug
description if needed, and if the list of changed files/functions has
changed, either updates the list (if there are no annotations), or
otherwise appends the new list below.

* Scripts/webkitpy/tool/steps/preparechangelog.py:
(PrepareChangeLog._resolve_existing_entry):
(PrepareChangeLog):
(PrepareChangeLog._merge_entries):
(PrepareChangeLog.run):
* Scripts/webkitpy/tool/steps/preparechangelog_unittest.py:
(PrepareChangeLogTest.test_resolve_existing_entry):
(make_entry):
(test_ensure_bug_url):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@140511 268f45cc-cd09-0410-ab3c-d52691b4dbfc