Ugh, that should be an opt-in thing. I've had trouble with Trac parsing commit messages as trac-wiki syntax, which is generally a great idea. The problem is occasionally you'll use markup characters in a semantically meaningful way, e.g. "Switch from `foo` to $(foo)", and rendering the markup will make the commit message confusing to read.
I'd be much more enthusiastic if there was some magical way for the local git command line to also support Markdown rendering/preview.
