Fix the bucket path for release uploads.

This was missing the /releases/ before the URL, causing releases to end
up in the wrong place.
In the github get_change() implementation, skip files without a patch.

We just got an error from reviews.reviewboard.org where a file in the tree
didn't have an associated patch. In this case, we want to just skip it rather
than returning a 500.

Reviewed at https://reviews.reviewboard.org/r/5020/
Release Review Board 1.7.19.
Merge branch 'release-1.7.x'