coverage: enable test coverage
Unbreak update_against_templates with goals

It was broken since b3edd98 as post_template_update and post_vc_update
signals no longer have ‘oldstats’ or ‘newstats’ args, so check for their
presence first.
update_against_templates: Add some debug output
pootle_app: Don't catch naked exceptions
pootle_app: Import pootle.scripts.hooks only once
pootle_app: Import pootle_misc.versioncontrol only once
update_against_templates: Rewrite the command to be closer to the metal