Handle Gitaly aborted merge due to branch update

What does this MR do?

Adds tests and a guard for the edge case that a merge looses a race with a concurrent branch update on the branch it wants to merge into.

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

Why was this MR needed?

We did not handle this edge case correctly in Gitaly.

Screenshots (if relevant)

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

What are the relevant issue numbers?

Closes gitaly#854 (closed)