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

subcmds/sync.py

@@ -44,7 +44,7 @@ try:
 except ImportError:
   multiprocessing = None
 
-from git_command import GIT
+from git_command import GIT, git_require
 from git_refs import R_HEADS, HEAD
 from main import WrapperModule
 from project import Project
@@ -310,7 +310,8 @@ later is required to fix a server side protocol bug.
     return fetched
 
   def _GCProjects(self, projects):
-    if multiprocessing:
+    has_dash_c = git_require((1, 7, 2))
+    if multiprocessing and has_dash_c:
       cpu_count = multiprocessing.cpu_count()
     else:
       cpu_count = 1