Since git-1.7.9 it is not permitted to use 'git archive --remote=...'
with arbitrary sha1 refs. Only named refs are allowed.
See this git commit in git :-) :
ee27ca4a7 archive: don't let remote clients get unreachable commits
add make-git-snapshot.sh to make a tarball from upstream git.
if %global gitcommit is defined on the top, a snapshot is being built.
don't BuildRequire auto-tools to build from release tarballs.