Add Repository#RevParse.

Implementation to add rev-parse to git2go.

I'd really appreciate some feedback in this direction, happy to start over :smile:
There's no need for `ptr` to be on the heap as it's only used within this function and it's an open struct, declaring it as

    var ptr C.git_revspec

and using it as

    ecode := C.git_revparse(&ptr, r.ptr, cspec)

though it stops being a pointer at that time, so a different name like `crevspec` might be in order.