ref: Split up RPC implementations
The "refs.go" file contains three different RPC implementations, which makes it really hard to find them. Furthermore, this makes it tough to implement support for SHA256 as you cannot just implement it for a single RPC and then remove the build tag from the corresponding test file.
Split the code into per-RPC files to address these shortcoming. This allows us to enable testing FindDefaultBranchName with SHA256.
Part of #5439 (closed).
Edited by Patrick Steinhardt