Add a "git rename" to help with - surprise surprise - renames

Add a "git rename" to help with - surprise surprise - renames

It's stupid.  We'd want to rename directories too, but this doesn't do
that yet - easy enough to do per se, we just need to carefully list all
the pathnames that got moved (and remember to ignore the files that
weren't tracked but are in the subdirectory that got moved).

Doing the directory case will require a bit more scripting.. Something like

	oldfiles=($(git-ls-files | grep '^$src'))
	newfiles=($(git-ls-files | sed ':^$src: s:^$src:$dst:'))
	mv $src $dst && git-update-cache --add --remove -- "${oldfiles[@]}" "${newfiles[@]}"

might do it, except it needs to be done right, and carefully.  Methinks
perl is probably better at this. Hint hint..