git-update-dirs: Add -n/--dry-run option

e863aca8-d563-11e2-9044-001f3b596ec9
git-update-dirs: Add -g/--ga-sync option

c21a8f1a-d565-11e2-8fb2-001f3b596ec9
"ga sync": Refuse to run "git-annex sync" if there's local changes

ba5c9c28-d56a-11e2-a7bb-001f3b596ec9