How do I control the automated commit message on a merge?

I am using git-svn. The SVN repository I'm dealing with has a standard comment convention I must follow. It is a simple static string of information that must appear on the first line of the commit message. Everything else can be free-form.

I figured out how to add a commit template to my gitconfig file. So for regular commits, I think I'm fine. But when Git does a merge (no conflicts), it auto-generates a commit message like:

        Merge branch 'master' of  /path/to/other/repo

How can I change this so my required string gets written at the beginning of the commit message. I played with the commit-msg and prepare-commit-msg hooks, but it didn't seem to get triggered in the merge case.

Also, having the prepare-commit-msg hook add text had the side-effect that in my normal commits, the message was always accepted, even when I wanted to abort (vi using :q!). In case I ever do need to use prepare-commit-msg, is there a way I can use it where abort still can work?

Thank you