magit-revert-buffer: remove incomplete comment

Remove half truth about why we are not reverting an indirect buffer.
Sure, as the comment said, that would revert the parent buffer; but
the reason we don't want that is relevant: we would end up doing it
twice.  We *could* extend the comment, but then again there are many
things we could add a comment for.
magit-revert-buffer: change order of two conditions

The conditions whose positions are switched are both rather cheap.
By switching them all the "unless reverting would destroy something"
are now located next to each other without an unrelated condition
in between.
magit-revert-buffer: use more appropriate conditionals
magit-revert-buffer: do not revert untracked files
Merge branch 'revert-buffer'