Update CE branch whenever EE is ahead of CE branch

What does this MR do?

Update CE branch whenever EE is ahead of CE branch

Example CE branch at: https://gitlab.com/gitlab-org/gitlab-ce/commits/ce-5960-consider-EE-ahead-of-CE

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

I completely understand that this is very hard to follow and understand, so please let me know if there's anything you're not clear of, I would be happy to explain for any details.

Why was this MR needed?

This should fix the last issue in https://gitlab.com/gitlab-org/gitlab-ee/issues/5960#note_72669536

Consider this scenario:

• Both CE and EE MRs created
• Both CE and EE got updated (making both CE and EE MRs behind of their corresponding master)
• Next CE upstream happened and merged
• Update EE MR with EE master, making the EE MR get commits from CE master, which aren't presented in CE MR yet

When this happened, we need to rebase CE branch against the newer CE merge base (not against CE master because there might have even newer commits weren't merged into EE yet), and if this would give any conflicts, the author should resolve the conflicts for CE branch first.

This is actually the same scenario when we're rebasing CE branch, but this time with EE branch rather than CE branch.

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer

What are the relevant issue numbers?

Closes #5960 (closed)