style: use string constants for refs/ paths
These were introduced in Dulwich 0.19.3, released in June 2018. We obtain most of these directly, but there's no constant for remotes, though, so we add our own.
These were introduced in Dulwich 0.19.3, released in June 2018. We obtain most of these directly, but there's no constant for remotes, though, so we add our own.