Allow user to configure "default" branch

[rzblue]

Some repositories don't use a main/master branch setup and instead have rolling release branches (e.g. named with the year major version). Currently wpiformat can't be run at all on those.

[auscompgeek]

Would defaulting to origin/HEAD be a bad idea?

[calcmogul]

That sounds like what we want according to https://superuser.com/a/1192881