This fixes a bug where repeated use of ** didn't behave as it should. In
particular, each use of `**` added a new requirement directory depth
requirement. For example, something like `**/**/b` would match
`foo/bar/b`, but it wouldn't match `foo/b` even though it should. In
particular, `**` semantics demand "infinite" depth, so repeated uses of
`**` should just coalesce as if only one was given.
We do this coalescing in the parser. It's a little tricky because we
treat `**/a`, `a/**` and `a/**/b` as distinct tokens with their own
regex conversions. We also test the crap out of it.
Fixes#1174
aeaa5fc1b1