GitHub import: Run housekeeping after initial import

Stan Hurequested to merge
sh-git-gc-after-initial-fetch into master

After an initial fetch, the repository will have many loose objects and refs. Running a git gc will compact refs into packed-refs and objects into .pack files, which should make importing pull requests faster.

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/59477

Merge request reports