Bump pygit2 to 0.20.3 

* merge with github/libgit2/pygit2  0.20.3 
* using cmake to build libgit2.a
* pygit2 build with ssl support
* alltests passed, with ellen(code version) tests passed
@xtao 这个没起作用呢。。vendor/libgit2 build之后还是dirty