This will improve integration with the rest of webkitpy, particularly when invoked via
webkit-patch.

Fixes <http://webkit.org/b/64115> REGRESSION (r90564): webkitpy's Checkout class uses
Executive inappropriately

Reviewed by Adam Barth.

* Scripts/webkitpy/common/checkout/checkout.py:
(Checkout.commit_message_for_this_commit): Use SCM.run instead of creating an Executive for
our own use. SCM might have some super-special Executive that it uses under the covers, and
we want to use it, too!

* Scripts/webkitpy/common/checkout/checkout_unittest.py:
(CommitMessageForThisCommitTest.test_commit_message_for_this_commit): Mock the SCM.run
method to call through to Executive.run_command.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@90584 268f45cc-cd09-0410-ab3c-d52691b4dbfc