During a project import, it's possible that new branches are created by
the importer to handle pull requests that have been created from forked
projects, which would increment the `pushes_since_gc` value via
`HousekeepingService.increment!` before a full garbage collection gets
to run. This causes HousekeepingService to skip the full `git gc` and
move to the incremental repack mode. To ensure that a garbage collection
is run to pack refs and objects, explicitly execute the task.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/59477