Git notifications for master

I'm just realizing that the logic the git-notifications script uses
for deciding when to send a diff, will often not produce one for
changes to master. That's because with a merge, the commit has
already been seen before and thus the assumption is that the diff
has already been mailed out.

I will change the script to always mail out diffs for everything
going into master, even if we have already seen it before.

Robin

... and once I have done that, I'll also send retrospective diffs
for the recent master changes for reference.

Robin