Fetching tarball of a commit

[Sean Farley]

> I do this in some local Portfiles, checking it now it's seems to get redirected, but it works.
>
> PortSystem          1.0
> PortGroup           github 1.0
>
> checksums           rmd160  38cf5c7de055057df1fc081d261e198d45127f3d \
>                    sha256  c866214e6c54731a8cf6a89b7c85b3c631db22db1228db63403ccf7aabca38cc
>
> github.setup MythTV mythtv 9ccfac11f31d8d05d48092efcbec8015c68f6cc1

As Ryan points out, this sha1 isn't easy to version with. I think I
can finish up the github group patch I was working on and do something
like this:

PortSystem          1.0
PortGroup           github 1.0

checksums           rmd160  38cf5c7de055057df1fc081d261e198d45127f3d \
                    sha256
c866214e6c54731a8cf6a89b7c85b3c631db22db1228db63403ccf7aabca38cc

github.setup        MythTV mythtv 0.25-84
git.branch          9ccfac11f31d8d05d48092efcbec8015c68f6cc1

Now, what to do with livecheck? There are two scenarios:

1) Keep the portfile versioned at the repo's HEAD
2) Keep the portfile versioned at a specific revision

In my patch, I've made (1) the default when git.branch is set. If (2)
is desired (which seems to be the case for MythTV), then an extra step
would be needed:

livecheck.type      none

If this is alright, I'll submit my patch today.