Why, exactly? It sounds like an oddly interesting idea to me, too, but I'm not sure why and I'd like to hear your reasons.
The recently-announced fork of the Plan 9 operating system, 9front, keeps it source in a Mercurial repository, and includes a Go compiler in the base operating system (which makes sense, since the creators of Plan 9 are also pretty much the creators of Go). I guess a Mercurial port to Go would make life a lot easier there.
Avoiding the overhead of python's startup time alone would be nice, hg is impressively fast for being written in Python, but could be much faster and memory-efficient in a compiled language where you have control over memory layout.
And that is without going into the potential of perhaps taking advantage of concurrency and parallelization for some things.
I'm actually considering doing this now you've said it. If there would be enough interest that is. There are very few "non-python" complete hg implementations.
I started tinkering with an implementation of revlog on the plane back from OSCON. If I get anywhere useful I'll post the sources somewhere.
Would be interested in that. Can you post back here if you do stick them somewhere.