checkupdates: exit with error if fdroiddata git repo is dirty

checkupdates: exit with error if fdroiddata git repo is dirty

One key security property of the F-Droid ecosystem is that the sensitive
code is all stored forever in git repos and source tarballs.  That means
we can easily go back and see if there where exploits and where they came
from.  Therefore, checkupdates should require everything in fdroiddata be
committed to git before running.

This provides --allow-dirty to override that behavior.