Support non-master default branch for boris-git.toml

[charleso]

Currently boris is hard-coded to read boris-git.toml from master, but if you change the default branch to something else that's not respected. This should be done in a non-github specific way, by using the remote HEAD ref.

[nhibberd]

This on affects where the boris-git.toml is read from correct?

[charleso]

This on affects where the boris-git.toml is read from correct?

Yep