A branch can't be a prefix (including a slash) of another branch. For
instance, you can't have a branch `a` and a branch `a/b`. Same with
tags. However, you can have a branch `a` and a tag `a/b`, which is
ambiguous.

In ExtractsRef, we do some work to handle these cases. That can involve
a fairly slow Redis SMEMBERS call. If a repository has no ambiguous refs
of this form (which should be the majority), we can perform some
optimisations.

For the purposes of this method, we only consider refs ambiguous before
the first slash. If we have a branch `a/b/c` and a tag `a/b`, this
method won't detect that.