[MERGE #102747] pre_commit hook

[Ian Clatworthy]

Ian Clatworthy has voted resubmit.
Status is now: Resubmit
Comment:
Everything looks fine to me here *except* the location in the code 
whether you're calling the pre-commit hooks. It looks like that's being 
done after the commit to the local branch which isn't what I (at least) 
expect of a pre-commit hook. Perhaps we should discuss the best location 
on #IRC?

Could you also update the very recently added hook documentation that 
Aaron has added?

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3Caac8d3110708270148m40f48573y1a55a6f3b6206e67%40mail.gmail.com%3E