Pass http_proxy as -c http.proxy on Mac OS X

The system libcurl library seems to ignore http_proxy on Mac OS
X systems. Copy the http_proxy environment variable (if set) as
`git -c http.proxy` whenever running a Git command.

Change-Id: I0ab29336897178f70b85092601f9fcc306dd17e1

git_command.py

@@ -155,6 +155,8 @@ class GitCommand(object):
         gitdir = project.gitdir
 
     command = [GIT]
+    if 'http_proxy' in env and 'darwin' == sys.platform:
+      command.extend(['-c', 'http.proxy=' + env['http_proxy']])
     if bare:
       if gitdir:
         _setenv(env, GIT_DIR, gitdir)