magit-rev-parse: accept any numbers of arguments

On the next branch this was done several months ago.  The callers where
then adjusted to take advantage of the new flexibility.  Here we only
change this function itself.  We do so to make the Org-Magit package
compatible with Magit's master branch again, see magit/org-magit#1.
Merge branch 'master' into next

Conflicts:
	magit.el