github tags containing "/"

[Don Lewis]

I'm working on a couple of new ports where I'm using fetching the
distfiles from github.  One quirk is that the tags start with a prefix
of "tags/v".  I'm setting PKGNAMEPREFIX=${PYTHON_PKGNAMEPREFIX}.

If I follow the example for in the Porters Handbook using
DISTVERSIONPREFIX=tags/v then the distfile gets named
${PORTNAME}-tags/v${PORTVERSION}_GH0.tar.gz,
which seems somewhat undesirable.  This seems to be a bit misleading
and risky because this port is just a python wrapper around another
library that also has the same PORTNAME.  That port hasn't been
converted to fetch from github, and when it does it seems like there
would be a chance of collisions.

I seem to get better results by setting GH_TAGNAME=tags/v${PORTVERSION}.
In that case, the distfile gets named
${GH_ACCOUNT}-${GH_PROJECT}-${PORTVERSION}-tags-v${PORTVERSION}_GH0.tar.gz.
This seems to be excessively verbose, but safer.

Thoughts?