.topmsg should be optional

[aspiers]

It's often too heavyweight to expect the user to write a .topmsg file; the combination of the topic branch name and the log messages for the "real" commits within that topic branch are often sufficient, especially when the topic branch only contains a single "real" commit.

That said, it's worthwhile keeping the functionality as long as it's optional - having a message which summarises the topic branch can be very useful when sharing more complex topic branches, and also for auto-generating patch series cover letters and github pull requests.

See also #38, in which I propose ditching the .topmsg file altogether.