New hook script: contrib/check-commit

[Matt Mackall]

After correcting one too many violations of line one of the contribution
check-list today, I hacked this up:

http://www.selenic.com/hg/file/1274ff3f20a8/contrib/check-commit

Augie's been threatening to write a bot to do this for years and now
I've made it trivial; spare yourself some robotic public shaming and
configure this today.

-- 
Mathematics is the supreme nostalgia of our time.