Also check for squash! and amend! (these are all anchored to the beginning of
the subject), and WIP and DROPME for good measure (but only if they occur in the
first line, otherwise it wouldn't let me merge this very commit :)
My local git is not compiled with PCRE support, so using -E makes it easier for
me to test the script locally. And -E is good enough for the simple matching we
want to do here.
