Move git operations for squash and rebase into Gitlab::Git

What does this MR do?

Move git operations for squash and rebase into Gitlab::Git

Are there points in the code the reviewer needs to double check?

For simplifcation, log messages are not exactly the same as they were before. This should have no other impact, I'm guessing.

Why was this MR needed?

We need to encapsulate git operations into Gitlab::Git for ease of migration to Gitaly.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Closes gitaly#761 (closed)