Experimental pushgate instead of patchbomb

[Matt Mackall]

If you set your default-push to:

http://selenic.com/repo/hg-push/

..changesets pushed to it will automatically be converted into an email
patchbomb to the list, with some basic checks applied.

Caveats:
- it automatically adds the STABLE flag based on the branch, so be sure
you get this right
- it has no way yet to set other flags like V2 or RFC
- there are probably some check-commit checks that'll be false positives

The primary goal here is to lower the configuration barrier for
first-time contributors. Please help test!

-- 
Mathematics is the supreme nostalgia of our time.