finally made `vc commit -F` (edit/fix/amend last commit) work

easiest way to do this was to make a separate command (`vc commit-fix`) and have commit's -F switch mutate itself into that
-	this was easier than expected; I just gave the `command` attribute a writer method
-	then, when execute() does its thing, it think it's a whole different command than how it was invoked
initial implementation of this functionality in Git
-	doing it in Subversion would involve doing an `svn propedit`, which would likely fail most of the time anyway
-	also, Subversion wants filenames, which Git neither wants nor needs, so that would be problematic to implement
-	and, again: how much do I really care about Subversion at this point?
also fixed the `vc commit` usage message while I was in there