Remove all warnings during compilation.

- Many PyLong_FromLong changed into PyLong_FromLongLong.
- A long used as out argument for git_config_parse_int64 changed into a
  int64_t.
- Many len variables changed from int into Py_ssize_t. Removed some
  castings related to those variables.
- Functions py_str_to_git_oid(_expand) return a Py_ssize_t, which is the
  return type of PyBytes_AsStringAndSize. Error values are usually casted to
  int, since the only error returned from those functions is -1.
- Changed RefLogIter i and size fields from int into size_t.
- Marked to_unicode_n and to_bytes inline functions as "unused". Not all
  compilation units which include utils.h use them.

Tested with:
$ clang --version
Apple clang version 4.0 (tags/Apple/clang-421.0.57) (based on LLVM 3.1svn)
Target: x86_64-apple-darwin12.3.0
Thread model: posix