Accessing the branch name in loginfo?

[Ben_Tilly]

I am trying to set up a script that does automatic actions when you check
the script in, and the action taken (re-running a template system) needs to
know the name of the branch the commit happened to.

How do I find that?

After some time with the documentation, the best that I can do from what I
can find is as follows.  I can use %v to get me the new version number.
>From the version number I can (with parsing in the target script) find the
largest odd number of ordinals separated by periods.  That is the revision
number of the branch.

However now I am stuck.  I can do all of this, but I don't know where the
revision number/name translation is to be found.

:-(What I really want is to find something like %b that I could slip into
loginfo, but no such luck so far...)-:

Thanks,
Ben