Importing pull requests from GitHub where the source branch was removed

ZD: https://gitlab.zendesk.com/agent/tickets/17353

Summary

Customer is worried that the pull requests count not matches the merge requests imported.

Description

This happens because the pull requests where the source branch was removed aren't imported. We also have some validations against the source branch, one inclusive that checks if the source branch exists when we create a new MR.

Suggestions

Do you have any idea that is possible to recover the removed branch (may be using reflog) or this is something we'll don't be able to do?

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/cc @balameb @DouweM @rspeicher