Fixed internal push state not being cleared when calling git_remote_download()

That's a recent regression due to the addition of the `git_remote_download()` API: if re-using the same remote to upload then download, the internal push state is still around and calling `git_remote_update_tips()` doesn't update the tags anymore.

BTW: I think there's another issue with `git_remote_download()`: the doc says it connects if needed, but I don't think it does looking at the implementation. `git_remote_upload()` does it for sure though. IMO neither API should auto-connect since they are low-level. For `git_remote_push()` and `git_remote_fetch()` however, it certainly makes sense.