Don't keep OBS state in git anymore

...just in OBS.  This is possible now that I understand enough
about OBS to write a _service file that will work directly on
the github tree and will extract all the packaging control files
it needs from there.  Now OBS contains only a fixed _service file
and git contains only the .spec and other packaging control files.
Switch spec file to tar.gz tarball

to match what the _service file does.
Tweak spec file to build from git snapshot tarball

instead of from proper release tarball.  We no longer assume
that the configure script or built documents exist.