Make Monotree Merging Magnificent

The following ideas are extracted from or based on http://blog.ffwll.ch/2017/08/github-why-cant-host-the-kernel.html.

A monotree describes a project that merges into a single Git repository but is developed in multiple forks. This is an approach the Linux kernel follows. For further explanation and advantages over other workflows please refer to the article linked above.

I have created subissues for most of the missing features below. Feel free to open new ones for those that I missed.

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forks

• multiple forks of same project within a namespace (https://gitlab.com/gitlab-org/gitlab-ce/issues/36268)
• restore/create forking relationship (https://gitlab.com/gitlab-org/gitlab-ce/issues/36269)
• create merge requests across forks (https://gitlab.com/gitlab-org/gitlab-ce/issues/20097)

issues

• cross-project / cross-namespace issues (https://gitlab.com/gitlab-org/gitlab-ce/issues/36272)
• issue status per project (https://gitlab.com/gitlab-org/gitlab-ce/issues/36272#note_37252760)

merge requests

• cross-project merge requests (https://gitlab.com/gitlab-org/gitlab-ce/issues/36273)
• merge request from e-mail (with patch) (https://gitlab.com/gitlab-org/gitlab-ce/issues/32878)
• subscribe to merge request diffs via e-mail
• reply to merge request diffs via e-mail (as we support for replying on comments)

e-mails

• Reply by Email - mbox/maildir support (gitlab-org/gitlab-ce#19524)
• In-Reply-To email header is incorrect (gitlab-org/gitlab-ce#36054)