Insert extern function 'oid_str_free' for free OID string that allocated...

This change made sice my application crush .
My application compiled on  win64 machine debug version.
My application use pygit2 (that use libgit2).
pygit2 allocate oid string using git_oid_allocfmt but free it by itself because git__free is not extern function.
As result of that beffer allocate and free with different compiled code.
To Solve that i insert new API ' oid_str_free' and call it form pygit2 instead free.