This is pretty useful in avoiding races: I want to create a ref only if it doesn't already exist. I can't check first because of TOCTOU -- by the time I finish the check, someone else might have already created the ref. And I can't take a lock because then I can't do the create, since the create expects to take the lock. The semantics are inspired by git update-ref, which allows an all-zero old value to mean that the ref must not exist.
Provide a function that can check reference name validity but can also signal when an error occurs. Use the name "name_is_valid", which is more suggestive of checking a given name, rather than "is_valid_name", which suggests that the function checks the validity of the current reference's name.
The C standard does not specify whether an enum is a signed or unsigned type. Obviously, any enum that includes negative values _must_ be signed, but if all values are positive then the compiler is free to choose signed or unsigned. Thus, by changing the type signatures to `git_object_t` and declaring the old `GIT_OBJ_` values as a signed or unsigned int, we risk a mismatch between what the compiler has chosen for a `git_object_t`'s type and our type declaration. Thus, we declare the deprecated values as the enum instead of guessing.
Move `git_ref_t` to `git_reference_t`