Expose GIT_DIRECTION constants; ability to remove refspecs

I've recently run into some trouble working with fetch/pull refspecs in pygit2 due to libgit2 features that haven't been exposed to python. The main issues I've encountered are:

• There's currently no way to remove fetch/pull refspecs
• The GIT_DIRECTION_FETCH/GIT_DIRECTION_PUSH constants aren't exposed to python (which are used by the Refspec type returned by get_refspec())

I found this old issue (https://github.com/libgit2/pygit2/issues/255) where @jdavid referenced these features, but it looks like those specific ones didn't make it in to pull request that was ultimately merged.

首先跟踪 Python 绑定中的 Refspec 类型和 get_refspec() 入口点，然后比较 libgit2 提供的 refspec 操作。当 GIT_DIRECTION_FETCH 和 GIT_DIRECTION_PUSH 得到暴露，并且可以移除 fetch/pull refspec，同时覆盖这两种行为时，这项工作就完成了。