D4040: shortest: make isrev() a top-level function

[Yuya Nishihara]

> +def mayberevnum(repo, prefix):
> +    """Checks if the given prefix may be mistaken for a revision number"""
> +    try:
> +        i = int(prefix)
> +        # if we are a pure int, then starting with zero will not be
> +        # confused as a rev; or, obviously, if the int is larger
> +        # than the value of the tip rev
> +        if prefix[0:1] == b'0' or i > len(repo.changelog):

`len(repo)` should be better since `repo.changelog` over repoview isn't
instant. Another option is to pass in an unfiltered repo as before.