Run housekeeping after initial import for all importers

The following discussion from gitlab-ce!26600 should be addressed:

• @stanhu started a discussion:

  We probably need to consider doing this line and the change_head call for all our importers.

# The initial fetch can bring in lots of loose refs and objects.
# Running a `git gc` will make importing pull requests faster.
Projects::HousekeepingService.new(project, :gc).execute