If you were issuing emails for two branches, and one merged the other,
you would get the same log messages appearing in two separate emails.
e.g. A working repository, where the last push to central was done at
the revision marked "B", after which two branches were developed
further.
* -- B -- 1 -- 1 -- M (branch1)
\ /
2 -- 2 -- 2 (branch2)
Now imagine that branch2 is pushed to the email-generating repository;
an email containing all the "2" revisions would be sent. Now, let's say
branch1 is pushed, the old update hook would run
git-rev-list $newrev ^$baserev
Where $newrev would be "M" and $baserev would be "B". This list
includes all the "2" revisions as well as all the "1" revisions.