Reduce code complexity on GitPushService#execute

Code complexity for gitlab-ce after this has been refactored:
```
  27.3: GitPushService#execute
```
This still needs to be merged into `gitlab-ee` presumably with conflicts... Perhaps we should create another issue for doing that?

I left the code sort of similar to what it was... If I could, I would have refactored most of the code into separate classes, etc. as it breaks probably all SOLID principles.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/13327


See merge request !2784