abdt_branch: optimise, avoid redundant marking
abdt_branch: optimise, avoid redundant marking

Avoid redundantly pushing a tracking branch in the case where a branch
is already marked 'bad_pre_review' and it doesn't have any new commits.

This path shows up fairly hot in the dashboard stats, later we may want
to cover all pushes with a check like this.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh
paid attention to 'unknown_user' test case

aevri/phabricator-tools,aevri/phabricator-tools,kjedruczyk/phabricator-tools,cs-shadow/phabricator-tools,bloomberg/phabricator-tools,aevri/phabricator-tools,kjedruczyk/phabricator-tools,bloomberg/phabricator-tools,cs-shadow/phabricator-tools,valhallasw/phabricator-tools,aevri/phabricator-tools,valhallasw/phabricator-tools,valhallasw/phabricator-tools,kjedruczyk/phabricator-tools,kjedruczyk/phabricator-tools,aevri/phabricator-tools,bloomberg/phabricator-tools,cs-shadow/phabricator-tools,valhallasw/phabricator-tools,kjedruczyk/phabricator-tools,bloomberg/phabricator-tools,cs-shadow/phabricator-tools,cs-shadow/phabricator-tools,bloomberg/phabricator-tools