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`
Since we now always build the library with cdecl calling conventions, our callbacks should be decorated as such so that users will not be able to provide callbacks defined with other calling conventions. The `GIT_CALLBACK` macro will inject the `__cdecl` attribute as appropriate.
Update the reference type names from an abbreviation (`git_ref`) to use the fullname (`git_reference`). This ensures that we are consistent with our naming for reference types and functions throughout the library. The previous names are now marked as deprecated.