I think of these messages as they appear to other developers. They don't yet have the changes applied, and there is the implicit question, "what will applying this changeset/patch do?" It will "Fix the XXX bug in YYY"!
For other verbs writing them as a command seems more natural, and works better if you have a specific goal up-front—you can literally write the commit summary along with up-front tests before the work is done.
I don't put a huge amount of weight on it, but for me this is the path of least resistance while maintaining consistentency.
