Use os.environ.copy() instead of dict()

Signed-off-by: Shawn O. Pearce <sop@google.com>
(cherry picked from commit 3218c13205)

git_command.py

@@ -127,7 +127,7 @@ class GitCommand(object):
                ssh_proxy = False,
                cwd = None,
                gitdir = None):
-    env = dict(os.environ)
+    env = os.environ.copy()
 
     for e in [REPO_TRACE,
               GIT_DIR,