gitlab: allow ignoring projects with certain path prefixes
Some GitLab instances use specific namespaces for transient repositories
that it doesn't make sense to archive (for example, gitlab.org has a set
of QA namespaces used for integration testing of their production
deployments; drupal has an issues/
namespace with forks of repos that
are only used for collaboration on merge requests, and aren't that
useful to be archived).
Related to #4680
Test Plan
added a test for the new flag
Migrated from D8918 (view on Phabricator)