Track a remote branch for an MSC

还没有人认领这个 Issue。

#### 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.

从 `@mscbot watch ` 命令以及 bot 现有的 MSC PR-branch 更新路径开始；跟踪远程分支和 fast-forward 更新的处理方式。完成标准是：已配置的远程分支可以在不进行 force-pushing 的情况下更新 PR 分支，包括托管在兼容 Git 服务器上的分支。

由索引模型根据 Issue 内容生成。