Fixes <http://webkit.org/b/26755> webkit-patch's commit messages are less readable than
commit-log-editor's

Reviewed by David Kilzer.

* Scripts/webkitpy/common/checkout/checkout.py:
(Checkout.commit_message_for_this_commit): Run commit-log-editor, passing it the paths of
the modified ChangeLogs, to generate the commit message, rather than trying to generate one
ourselves.

* Scripts/webkitpy/common/checkout/checkout_unittest.py: Updated the expected commit message
to match commit-log-editor's format.
(CommitMessageForThisCommitTest.setUp): Write the ChangeLogs into Tools and LayoutTests
directories so we can see how the various entries get labeled in the commit message.
(CommitMessageForThisCommitTest.test_commit_message_for_this_commit): Create a mock SCM
instance that knows how to find commit-log-editor and pass it to our Checkout instance.
Don't bother capturing output, since there shouldn't be any.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@90564 268f45cc-cd09-0410-ab3c-d52691b4dbfc