Support merge requests from forks

For Community contribution merge requests in gitlab-org/gitlab coming from a fork, the pipeline currently fails with:

fatal: reference is not a tree: fc674193530a9ac9bcc5cf13da4ef8ef007c5a84

Example: https://ops.gitlab.net/gitlab-com/database-team/gitlab-com-database-testing/-/jobs/4983874

This is because it assumes the reference to be in the original gitlab-org/gitlab project and we can't parameterize this.

In order to also be able to test migrations from forks, in particular for JiHu contribution , we'd like to parameterize the pipeline with the project to retrieve the SHA from.

Edited Sep 29, 2021 by Andreas Brandl

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