Add git hooks: (1) auto-update; (2) prevent commits; (3) reminders about git-integration

[johnkeeping]

Are you aware that git integration --rebuild integration does not need to be executed with the "integration" branch checked out? Yes, "integration --rebuild" is slightly longer to type than "checkout" but it's still just one command and it doesn't force you to go through a rebuild just because you're moving branches - don't forget that rebuilding the integration branch potentially involves touching lots of files which can make a big difference to the time taken for incremental compilations.

I'm not sure how your second suggestion will help anyone. Since there's no way to install hooks on clone (for good reason), there would have to be documentation somewhere about setting them up; since that's per-project overhead anyway it seems simpler to just tell contributors "don't develop on top of the 'pu' branch". At which point either the user really is unsuspecting, in which case they won't have installed the hook anyway, or they have read the document and don't need the hook.

AFAIK there is no way in Git to provide a message on first clone, with the possible exception of using some wrapper (like Gitolite) or alternate server software and even then identifying a first clone is difficult.

I'm not averse to merging something that adds some hooks that people find useful into contrib/hooks/ but I don't intend to implement any of these myself.