sync: Only parallelize gc for git >= 1.7.2

This minimum version is required for the -c argument to set config on
the command line. Without this option, git by default uses as many
threads per invocation as there are CPUs, so we cannot safely
parallelize without hosing a system.

Change-Id: I8fd313dd84917658162b5134b2d9aa34a96f2772
1 file changed