Handle base branch changes

[pietroalbini]

The current homu codebase doesn't listen for base branch changes, so if a PR is created targeting master and then switched to target beta (like rust-lang/rust#57670), the PR is going to be merged on master.

This PR makes three changes to homu:

• Add a check before starting a test to make sure the base branch is the same recorded by homu
• Listen for base branch changes and update the homu state
• Unapprove the PR if it was previously approved, and stop testing it if it's already testing. This prevent malicious actors from making a PR targeting beta and then switching it to target stable after the reviewer approved it.

I also added a few lines to update PR title and body when it changes, since I was changing this part of the code anyway.

r? @alexcrichton or @kennytm
cc @Manishearth @jdm, you probably want to cherry-pick this on servo's homu