Add a stronger safety belt to this script by comparing the actual
PORTREVISION (using the Ports Collection framework, not just looking at
one Makefile individually at a time) before and after the bump. If the
version after the bump isn't actually increased, flag that as an error.

As an example, before revision r464215 (cf. bug #226533) this script
would have wreaked wreak havoc on the multimedia/avidemux* ports.
This hardly can be blamed on bump-revision.sh, but with the additional
safety belt it does now detect such cases.

How to reproduce:

% cd $PORTSDIR
% svn up -r 464036 multimedia/
% Tools/scripts/bump-revision.sh multimedia/avidemux*

With this patch we print:

INFO: multimedia/avidemux PORTREVISION=	9 found, bumping it by 1.
INFO: multimedia/avidemux-cli PORTREVISION not found, adding PORTREVISION= 1
ERROR: multimedia/avidemux-cli PORTREVISION went backwards from 5 to 1!
INFO: multimedia/avidemux-plugins PORTREVISION not found, adding PORTREVISION= 1
ERROR: multimedia/avidemux-plugins PORTREVISION went backwards from 5 to 1!
INFO: multimedia/avidemux-qt4 PORTREVISION not found, adding PORTREVISION= 1
ERROR: multimedia/avidemux-qt4 PORTREVISION went backwards from 5 to 1!

The beauty of this approach is that it goes beyond a simple text search,
and leverages what the ports framework itself does.

PR: 226926, 226533
Approved by: maintainer timeout (20+ weeks)
Reviewed by: mandree, riggs