Create rake task that will run git repack -a on all repositories

This is the result of the discussion in https://gitlab.com/gitlab-org/omnibus-gitlab/issues/510#note_1116496

Summary:

git 2.x came with performance improvements. These improvements are on by default for servers using git 2 but it is possible that some old repositores would benefit from git repack.

According to Dmitriy's comment

1. Create the rake task
2. Run on dev
3. Run on gitlab.com
4. Add to blog post for 7.11

cc @dzaporozhets @DouweM @rspeicher @vsizov @job @patricio @sytses

Milestone removed

Milestone changed to 7.12

Milestone changed to 7.13

Did this happen?

@kjeremy No

Milestone changed to 8.1

mentioned in merge request !1388 (merged)

mentioned in merge request !1388 (merged)

Status changed to closed by merge request !1388 (merged)

mentioned in commit a80f0f66