Implement tag and reference overwriting

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Bind libgit2's methods

GIT_EXTERN(int) git_tag_create_f(
        git_oid *oid,
        git_repository *repo,
        const char *tag_name,
        const git_oid *target,
        git_otype target_type,
        const git_signature *tagger,
        const char *message);

GIT_EXTERN(int) git_reference_create_oid_f(git_reference **ref_out, 
                         git_repository *repo, const char *name, const git_oid *id);

GIT_EXTERN(int) git_reference_create_symbolic_f(git_reference **ref_out, 
                        git_repository *repo, const char *name, const char *target);

Change signature of the Create() methods of TagCollection into

public Tag Create(string name, string target, Signature tagger, 
             string message, bool allowOverwrite = false);

public Tag Create(string name, string target, bool allowOverwrite = false);

Change signature of the Create() methods of RefCollection into

public Reference Create(string name, string target, bool allowOverwrite = false);

public Reference Create(string name, ObjectId target, bool allowOverwrite = false);

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Fixed by PR21