UserCreateBranch: unify API responses between Go & Ruby response paths
Change code added in [1] to use GetReference() instead of GetBranch(), and make it return the same error messages & codes as the Ruby code.
This resolves numerous bugs in [1] which weren't spotted due to inadequate test coverage. Those are now fixed.
Other bugs in the code added in [2] remain, but I'm still working on fixing those.
See also [3] for the initial discarded MR to fix issues in this code for both UserCreate and UserDelete at the same time (that WIP itself has bugs...).
- c1e3ccca (Initial go implementation of UserCreateBranch, 2020-09-30)
- c3b32722 (Initial go implementation of UserDeleteBranch, 2020-09-29)
- !2838 (closed)