b4: introduce configuration for the Git project

In the preceding commit we have extended our documentation to recommend b4 for sending patch series to the mailing list. Introduce configuration so that it knows to honor preferences of the Git project by default. For now, this configuration does two things: - It configures "send-same-thread = shallow", which tells b4 to always send subsequent versions of the same patch series as a reply to the cover letter of the first version. - It configures "prep-cover-template", which tells b4 to use a custom template for the cover letter. The most important change compared to the default template is that our custom template also includes a range-diff. There's potentially more things that we may want to configure going forward, like for example auto-configuration of folks to Cc on certain patches. But these two tweaks feel like a good place to start. Note that these values only serve as defaults, and users may want to tweak those defaults based on their own preference. Luckily, users can do that without having to touch `.b4-config` at all, as b4 allows them to override values via Git configuration: ``` $ git config set b4.prep-cover-template /does/not/exist $ b4 send --dry-run ERROR: prep-cover-template says to use x, but it does not exist ``` So this gives users an easy way to override our defaults without having to touch ".b4-config", which would dirty the tree. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>