Create a vc_git-compliant update hook that will not OOM when trying to update all repository default branch info

@dww - yes, it could and will be useful in the future. for sure. unfortunately figuring out a generic pattern for communicating with them is a little tricky, so we're just gonna one-off it here.

@DamZ - direct repository read, and database write. from the update hook, we need to enqueue one job for each of the 19000+ repositories. that job reads the current symbolic ref in HEAD from disk, then writes it to the db. preferably, we would also verify that the master branch still exists, as it does not for a number of repositories, then pick another of the available branches (it wouldn't be hard to come up with a rough canonicality metric), and set that as the default if master has been deleted, THEN update the db accordingly.

at least, that's what we have to do to be complete about it. if we were to skip the latter step, it would be pretty easy to just grep all .git/HEAD files for anything which is not set to master (only a scant few are, core included), write 'master' to the db for the rest, then just let people sort out re-setting their own default branches accordingly. i would prefer not to do that, though, since we'd be knowingly creating an inconsistent data state that the application can't normally create.

ultimately the concern is that simply loading 19000 fully-classed repository objects, then running operations directly on them within the update function, could run OOM. popping up the memory limit might be enough of an answer to that question for us to do it sloppily without enqueueing, but we'll need to figure out just what the memory cost is. and to dww's original point, i don't want to have to artificially increase the memory limit every time we run an operation against all repos.

oh cool, that helps. handy that we don't use packed refs (yet), so we can still get away with just reading what's under refs/. we don't need the full branch list, only whether or not the current default branch exists, as vcapi already has all that info, and can also access the info on whether or not a branch has a release associated with it. we need that info to make the smartest decision about what to set as the new default branch. my preferred criteria would be, in decreasing importance, a) that the branch has a release, b) that the branch is for D7, and c) that the branch has the highest available major version number. we've got a nontrivial number of repos to do this for:

grep '^project.*master;false' git-branch-data.txt | wc -l
1493

just about as many sandboxes to do, though the first criteria doesn't apply there.

grep '^sandbox.*master;false' git-branch-data.txt | wc -l
1757

we'll still need to enqueue jobs for all those repositories which need to have that their default branch updated, but since we'll have a full list of all the repos to hit, we'll be able to single-load them all and enqueue the jobs, so no risk of memory explosion.

the list of repos which have non-master branches as the default is much smaller, and we can just manually map those.

grep ';true' git-branch-data.txt | grep -v master | wc -l
9

hard part's done though, thanks Damien - i can take care of the update hook.

heh, turns out the mem usage isn't actually that bad - loading all 19k repos at once eats about 90MB. oh well :) then again, that's just loading the repos, not loading all of their branches or anything. doesn't obviate the need for a better strategy on this in the long term, either. some sort of multi-part or batch strategy.

commushed, with a hardcoded path to a local version of that file. once we're ready to deploy, i'll regenerate the file so that it's as up to date as possible then ensure it's in place.

marking postponed so that we don't forget to do that on deploy.

nope, intentionally left it out. i'm going to generate it just before we start and drop it in the repo then.

meh, actually, no real harm putting it in now. commushed the script and a current data dump in there. we can remove them from the repo after this update is done.