GitFusion mirroring support

Description

A customer has asked GitLab to support bi-directional mirroring with Perforce GitFusion.

Proposal

• Prerequisite: improvements to mirroring to better handle bidirectional mirroring #3745 (closed)
• setup test instance for experimenting with GitFusion #3902 (closed)
• GitFusion specific tweaks (pending testing on test instance)

Links / references

https://gitlab.com/gitlab-org/gitlab-ee/issues/2396

Documentation blurb

Overview

What is it? Why should someone use this feature? What is the underlying (business) problem? How do you use this feature?

Use cases

Who is this for? Provide one or more use cases.

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.

• Feature assurance
• Documentation
• Added to features.yml