Track a remote branch for an MSC

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#### Is your feature request related to a problem? Please describe.
In situations where the original author may want to delegate an MSC, and where - due to security reasons, or likewise - the author is unable to grant the delegate author access to the PR's origin repository, it may want to have the PR branch be updated from a remote branch instead, such as the delegate author's own fork.

In other situations, an organisation may want to - for legal reasons, or likewise - develop the commits on their own git server, but have them be fetched to the MSC's PR branch.

#### Describe the solution you'd like.
For these problems, I'd like the mscbot to be able to be set up to watch a remote branch. Be it on github, or on any compatible git server.

The original author of the PR has to setup the watch, via `@mscbot watch `.

After this, the MSC bot will only try to fetch commits from that branch via a `git pull`-like mechanism. That is to say; it will not force-push the PR branch with updates from the remote branch, it will only append new commits that were present in the remote branch, but not in the PR branch.

In other words, the MSC bot will only [fast-forward](https://stackoverflow.com/a/29673993/8700553) the PR branch to the remote branch.

Commencez par la commande `@mscbot watch ` et par le chemin existant du bot pour la mise à jour de MSC PR-branch ; suivez la manière dont sont gérées les branches distantes et les mises à jour fast-forward. C’est terminé lorsqu’une branche distante configurée peut mettre à jour la branche du PR sans force-pushing, y compris les branches hébergées sur des serveurs Git compatibles.

Rédigé par le modèle d'indexation à partir du texte de l'issue.