Run housekeeping after initial import for all importers
The following discussion from gitlab-ce!26600 should be addressed:
- [ ] @stanhu started a [discussion](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26600#note_158179493):
> We probably need to consider doing this line and the `change_head` call for all our importers.
```ruby
# 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
```
issue