Track a remote branch for an MSC

まだ誰も着手していません。

#### 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.

`@mscbot watch ` コマンドと、ボットに既存する MSC PR-branch の更新経路から始め、リモートブランチと fast-forward 更新がどのように処理されるかを追跡します。設定されたリモートブランチから、force-pushing なしで PR ブランチを更新でき、互換性のある Git サーバーでホストされているブランチも対象にできれば完了です。

索引モデルが issue の本文から書いたものです。