Added missing "(require 'cl)".

As far as I can tell this fixes the following issues in the original
repository [https://github.com/rafl/git-commit-mode] issue tracker:

* "Fix git-commit-mode loading errors. #7"
  [https://github.com/rafl/git-commit-mode/pull/7]

* "Add missing require of cl #8"
  [https://github.com/rafl/git-commit-mode/pull/8]