Fix SPACK-87: Tag checkout fails for older git versions.
Fix SPACK-87: Tag checkout fails for older git versions.

skosukhin/spack,TheTimmy/spack,EmreAtes/spack,mfherbst/spack,krafczyk/spack,skosukhin/spack,mfherbst/spack,tmerrick1/spack,matthiasdiener/spack,krafczyk/spack,lgarren/spack,mfherbst/spack,LLNL/spack,tmerrick1/spack,lgarren/spack,iulian787/spack,tmerrick1/spack,LLNL/spack,skosukhin/spack,tmerrick1/spack,TheTimmy/spack,EmreAtes/spack,iulian787/spack,TheTimmy/spack,iulian787/spack,lgarren/spack,EmreAtes/spack,krafczyk/spack,lgarren/spack,LLNL/spack,mfherbst/spack,LLNL/spack,matthiasdiener/spack,lgarren/spack,mfherbst/spack,iulian787/spack,EmreAtes/spack,krafczyk/spack,skosukhin/spack,TheTimmy/spack,TheTimmy/spack,matthiasdiener/spack,iulian787/spack,skosukhin/spack,tmerrick1/spack,matthiasdiener/spack,krafczyk/spack,EmreAtes/spack,matthiasdiener/spack,LLNL/spack