Add contrib/stg-whatchanged: look at what would be changed by refreshing.

Add contrib/stg-whatchanged: look at what would be changed by refreshing.

This script outputs a "metadiff" (diff of diffs, ie. diff between 2
versions of a single patch).  This is somewhat a proof of concept, and some
work is indeed needed in the way to present the results.  Consider
filtering the output at least through colordiff.

I have 2 uses for this script:
- when simply editing a patch, provides a 3rd way to check what I've done
  before refreshing (in addition to "stg diff" and "stg diff -r //bottom")
- most usefully, when resolving conflicts caused by a push, to ease
  checking of the merge result.

Signed-off-by: Yann Dirson <ydirson@altern.org>