Unreviewed, fix simple mistake in r143903.
<http://webkit.org/b/110746>

Reviewed by Andreas Kling.

Pass git revision, not svn revision, to git shell command.

* Scripts/webkitpy/common/checkout/scm/git.py:
(Git.timestamp_of_latest_commit):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@143918 268f45cc-cd09-0410-ab3c-d52691b4dbfc