Add a new `upstream_merge` task

The goal is to have the task:

1. Merge CE into EE
2. Open a merge request
3. (Optional) Ping the most relevant people by looking at the conflicting files' history
4. Specs

Pre-requisites

1. Add the ability to find/create a merge request: !139 (merged)
2. Move remotes to the Project classes and get rid of the Remotes class: !168 (merged)
3. Add head, status, log, fetch, checkout_new_branch, pull, push, and merge to RemoteRepository: !177 (merged)
4. Introduce a new CommitAuthor class : !197 (merged)

Todo

1. Allow to pass MENTION=true (default to false) to ping people in the MR description

Original description

This is a test for !86 (closed)

The diff seems fine here, but the original one is in a pretty weird state. See !86 (diffs, comment 47024559)