Add extra logging to githost.log when rebasing

What does this MR do?

Adds some extra logging to help debug issues like those in https://gitlab.com/gitlab-org/gitlab-ee/issues/5966.

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

I don't think so!

Why was this MR needed?

We don't know what the underlying problem is.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer