Correctly support branch names with slashes

Fixes #250.

Previously, branches with slashes in the name would consume all slashed
segments in a URL, causing the routes to capture incorrect file paths.

This solution is not particularly elegant - anywhere a route used
both a commit-ish identifier and a path, we collapse those two params
into a single param, and parse that param inside the route.

It seems to be working reasonably reliably, but has not seen extensive
testing.

It is also not particularly pretty. If anyone sees ways to improve it,
please, have at it.