compare: cleanup of as_form handling

as_form is only used when loaded 'partial' from pullrequest.html. Including it
in the 'Detailed compare view' url did thus not do any harm.

as_form is never used in pullrequest_show.html or compare_diff.html and it is
thus never used together with swap_url ... which is why the wrong
'as_form=None' or 'as_form=False' in urls didn't do any harm.