Fix name colision when importing GitHub pull requests from forked repositories

What does this MR do?

Prefix with namespace and project path, branches from a forked repository to prevent branch name collision (master and master from different repositories).

Proposed pattern: pull/123/company/otherproject/master

See: #29034 (closed)

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

Do we have any other pre-defined pattern to follow on the branch naming?

Why was this MR needed?

Import was not working for PRs with branches using the same name on source and target repositories.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #29034 (closed)

CC @DouweM @xiaogang_gitlab