git-update-dirs: Disable ga repair, takes too long and is too expensive

Don't know how long this will last, it would've been nice to have it in
place. But the only corruption I've seen in git-annex repos has happened
after I've used --prune=now with git gc, and in those cases deleting
.git/annex/index has always fixed the problem.

a1884880-3033-11e4-abf8-fefdb24f8e10
git-update-dirs: compress: Remove --prune=now if should('dangling')

It's not necessary in most cases, and it has a tendency to mess up
git-annex repositories anyway.

6e78cdf6-3034-11e4-a188-fefdb24f8e10