I'm pretty disappointed by the performance of regex sets. They are
apparently spending a lot of their time in construction of the DFA,
which probably means that the DFA is just too big.
It turns out that it's actually faster to build an *additional* normal
regex with the alternation of every glob and use it as a first-pass
filter over every file path. If there's a match, only then do we try the
more expensive RegexSet.
