Update last commit message when merging with links to the PR and its title

[PetarKirov]

When dlang-bot does auto-merge-rebase or auto-merge-squash, it should update the message of the commit that the branch head points to with information like:

Merged-on-behalf-of: <reviewer>
Pull-Request: <url>
Pull-Request-Title:<text>
Pull-Request-Description: <text>

[CyberShadow]

Pull-Request:

The pull request number should always be on the first line. (This wasn't the case when the other merge methods were first introduced.)

Pull-Request-Title:

The first line should already be the pull request title.

Pull-Request-Description:

The description often contains information for reviewers, and thus usually doesn't need to be included in Git history.

Have I missed anything?

See also #181 and #66.