Add link to PR to mail footer

[sschuberth]

To get more context and easily browse the source code, it'd be great if the sent mail would contain a link to the PR and / or to the source branch of the PR in the footer. I imagine this to look similar to @dscho's footer (see e.g. [1]) that I find quite useful:

base-commit: e7e07d5a4fcc2a203d9873968ad3e6bd4d7419d7
Published-As: https://github.com/dscho/git/releases/tag/time_t-may-be-int64-v1
Fetch-It-Via: git fetch https://github.com/dscho/git time_t-may-be-int64-v1

[1] http://public-inbox.org/git/cover.1488231002.git.johannes.schindelin@gmx.de/