Conditionally create new direct reference

A direct reference (also called an object id reference) refers directly to a specific object id (a.k.a. OID or SHA) in the repository. The id permanently refers to the object (although the reference itself can be moved). For example, in libgit2 the direct ref "refs/tags/v0.17.0" refers to OID 5b9fac39d8a76b9139667c26a63e6b3f204b3977.

The direct reference will be created in the repository and written to the disk. The generated reference object must be freed by the user.

Valid reference names must follow one of two patterns:

1. Top-level names must contain only capital letters and underscores, and must begin and end with a letter. (e.g. "HEAD", "ORIG_HEAD").
2. Names prefixed with "refs/" can be almost anything. You must avoid the characters '~', '^', ':', '
   ', '?', '[', and '*', and the sequences ".." and "@{" which have special meaning to revparse.

This function will return an error if a reference already exists with the given name unless force is true, in which case it will be overwritten.

The message for the reflog will be ignored if the reference does not belong in the standard set (HEAD, branches and remote-tracking branches) and and it does not have a reflog.

It will return GIT_EMODIFIED if the reference's value at the time of updating does not match the one passed through current_id (i.e. if the ref has changed since the user read it).