Make argument parsing a bit more robust

Currently, when running mfh with a flag like --help the user gets a confusing
error message from expr(1) like the following:

> expr: illegal option -- -
> expr: usage: expr [-e] expression
>
> revision "--help" should be a number

This is not helpful. Instead, make sure that any arguments specified by the
user are handled properly by expr(1). This results in a much cleaner error
message:

> revision "--help" should be a number