Makes --revision-range behave like documented for one revision in git

I'm not sure this absolutely works in all cases, e.g. with merge requests, but I would definitely update it if anyone found problems. Commit message is listed below explaining the change.

> The documentation states that "If you only need to post a single revision,
> you can type: $ rbt post --revision-range=REVISION", however that was not the
> case. This changes makes rbt post --revision-range=REVISION act like you had
> given it --revision-range=REVISION^:REVISION which is how the documentation
> makes it sound like it should behave.
>
> NOTE: this one changes the behavior for git. Same behavior is apparently
> present for other tools.