build-release-tarball: Verify full releases are tagged and pushed.
build-release-tarball: Verify full releases are tagged and pushed.

This ensures that anything that looks like a full release will
identify itself as such.  It requires that the version have a tag at
the current commit (though it doesn't need to be pushed yet), as well
as the commits exist in `main` or a release branch in the remote.

rht/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,rht/zulip,rht/zulip,andersk/zulip,andersk/zulip,andersk/zulip,andersk/zulip,rht/zulip,zulip/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip