ExtractsRef attempts to take a string containing slashes and figure out
which part's a reference to a tree-ish object, and which part's a path
in that tree-ish. For instance, `foo/bar/baz` might mean:

1. A ref of `foo`, a path of `bar/baz`
2. A ref of `foo/bar`, a path of `baz`
3. A ref of `foo/bar/baz`, and no path (the root)

To do this, it needs to load the full list of references (branches and
tags) for the repository, which can be slow and put pressure on Redis as
it uses SMEMBERS, which is an O(n) operation.

When there is no slash in the string, there is no ambiguity: it must be
a reference and no path (the third case above). In that case we can skip
loading the references entirely and just detect that by looking at the
string in question.