Support creating merge requests from `git send-email`

When we support applying patches from attachments when creating merge requests from emails (https://gitlab.com/gitlab-org/gitlab-ce/issues/40830). Git supports sending changes via email using the git send-mail command, but it includes the patch as text at the bottom of the email rather than as attachments.

GitLab should support the native email output of git send-mail.

Proposal

Add support for opening a merge request from emails created git send-email

Links / references

• https://git-scm.com/docs/git-send-email
• https://git-send-email.io