Add depends_on('perl') to git package
Add depends_on('perl') to git package

This commit changes the git package to depend_on('perl').  The system
perl is not always sufficient to install git (e.g. a CentOS7 system with
the development tools group installed has perl but not the
ExtUtils::MakeMaker package that git needs) and one can't always update
the system's perl.

This PR depends_on PR #1339, which adds a perl package to spack.

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