Let GitGitGadget "read" the "What's cooking" mails

[dscho]

The Git maintainer sends out these mails (example), typically twice a week, that describe the current state of all contributions (at least those that did not fall between the cracks). The expectation is that the contributors (who are usually Bcc:ed) read the entire mail and figure out what parts concern them. It can be daunting for new contributors to even know that this is expected.

Let's teach GitGitGadget to parse those mails (actually, we can use the whats-cooking.txt file in the todo branch that we mirror into gitgitgadget/git anyway), and add PR comments to the GitGitGadget PRs about which there was talk in that "What's cooking" mails.

[dscho]

This has been long implemented, in #458 (and merged four months ago).