We were erroneously neglecting to prefix a pattern like `foo/`
with `**/` (to make `**/foo/`) because it had a slash in it. In fact, the
only reason to neglect a **/ prefix is if the pattern already starts
with **/, or if the pattern is absolute.
Fixes#16, #49, #50, #65
I don't like having multiple flags do the same thing, but -u, -uu and -uuu
are much easier to remember, particularly with -uuu meaning "search
everything."
We do this by avoiding using a RegexSet (*sigh*). In particular, file
type matching has much simpler semantics than gitignore files, so we don't
actually need to care which file type matched. Therefore, we can get away
with a single regex with a giant alternation.
