Resolve duplicate merge request iids during GitHubImport
What does this MR do and why?
This fixes the possibility for duplicate merge request iids being inadvertently created when new merge requests are raised/created during an ongoing GitHubImport operation.
Changelog: fixed
How to set up and validate locally
Numbered steps to set up and validate the change are strongly suggested.
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Related to #403330 (closed)