Extract git version check code from gclient_scm.py into scm.py where it belongs.

scm.py

Index: scm.py
diff --git a/scm.py b/scm.py
index 1dce4bd7ccd9d35dd6e3a8b09788c70c03a8036d..b4553765ae1fa248c85ffe56836e6e8fa336a7cb 100644
--- a/scm.py
+++ b/scm.py
@@ -270,6 +270,24 @@ class GIT(object):
     root = GIT.Capture(['rev-parse', '--show-cdup'], path)[0].strip()
     return os.path.abspath(os.path.join(path, root))
 
+  @staticmethod
+  def AssertVersion(min_version):
+    """Asserts git's version is at least min_version."""
+    def only_int(val):
+      if val.isdigit():
+        return int(val)
+      else:
+        return 0
+    current_version =  GIT.Capture(['--version'])[0].split()[-1]
+    current_version_list = map(only_int, current_version.split('.'))
+    for min_ver in map(int, min_version.split('.')):
+      ver = current_version_list.pop(0)
+      if ver < min_ver:
+        return (False, current_version)
+      elif ver > min_ver:
+        return (True, current_version)
+    return (True, current_version)
+
 
 class SVN(object):
   COMMAND = "svn"