split: remove an unnecessary (and confusingly typed) fallback revision
`opts.get('rev') or '.'` is either a list of strings or just a
string. It happened to work because `'.'[0] == '.'` on Python 2, but
it won't work on Python 3 (for byte strings). The fallback value
wasn't even needed (it was also set just after), so let's just remove
it.