[MERGE] Add a way to specify a template commit message

[Robert Collins]

Robert Collins has voted +1 (conditional).
Status is now: Semi-approved
Comment:
The check for 'start_message != ""' seems wrong to me - an empty start 
message is still a start message. Just checking for None appears 
sufficient to me. Other than that, the test for using start_message 
should be a separate test function, not an extension of the current one 
- its a new test, and the mutter() call seems entirely spurious to me.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C20070203140630.GC3641%40jameswestby.net%3E