release script doesn't like empty commits

[marten-seemann]

If a commit is empty, git log doesn't print the stats: https://github.com/libp2p/go-libp2p/blob/master/scripts/mkreleaselog#L52. As only automated commits issued by IPDX seem to be empty (why commit if you have nothing to commit...), this filter for the git log command can be used as a workaround: --perl-regexp --author='^((?!GitHub).*)$'.