misc quickfixes to scopes

@rymai - I found some bugs after publishing 2.7

Overall, I'll have to create a separate `::Guard::Scope` class with a *very* clear interface and specs about how the scope should be "compiled" based on the command line, guardfiles, scope set through pry, default groups, and guardfile reloading, etc.

There is one spec which I've marked as pending, since I don't know how to fix it - since I don't know e.g. if reevaluating a guardfile with a scope should override the commandline or not.

Anyway, I'll leave this open - but if a quickfix is necessary on master, this can be merged until I rework the scope handling code.