github portgroup release tarball url

[Jean-Philippe Ouellet]

Hello,

In the github portgroup, the urls it uses to fetch releases are:
	${github.homepage}/releases/download/${git.branch}
and
	${github.homepage}/tarball/${git.branch}

which fetches a tarball that extracts to
	${author}-${project}-${git_hash}/
which we then need to fix in post-extract.


The tarball for the release linked to from the github web interface is
	${github.homepage}/archive/${version}.tar.gz
which extracts to
	${project}-${version}/
which I think would be cleaner. (and is used in the majority of brew ports)


I found this issue while wondering why I kept getting checksum
mismatches in macports when it worked perfectly in homebrew.