Added 3  rake tasks for repository maintainance

## What does this MR do?
This MR adds 3 rake tasks

- gitlab:git:repack
  - `-a`
  - `--quiet`
- gitlab:git:gc
  - `--auto`
  - `--quiet`
- gitlab:git:prune
  - Needs git version > 1.8.4.1, Ubuntu repos @ 1.9.x

## Are there points in the code the reviewer needs to double check?
AFAIK this MR abides by the Guidelines for shell commands. Also, the output given is not the diskspace saved etc, just if the commands were succesfull. (Parsing output etc did not seem like the way to go)

Output might be verbose when a lot of repo's are in the system?

## Why was this MR needed?
`git gc` and `git prune` can reduce storage space used.

## What are the relevant issue numbers / Feature requests?
Closes #1529

## Screenshots (if relevant)
`rake -T`
![Screenshot_from_2015-09-22_14-57-59](https://gitlab.com/zj/gitlab-ce/uploads/4abfa00ce7afcc73f553d92581246731/Screenshot_from_2015-09-22_14-57-59.png)
(git fsck now removed as IMHO it doesn't add any value.)

See merge request !1388