Set GIT_ALLOW_PROTOCOL to limit dangerous protocols

See git commit 33cfccbbf35a -- some protocols allow arbitrary command
execution as part of the URL. Instead of blindly allowing those,
whitelist the allowed URL protocols unless the user has already done so.

Bug: Issue 210
Change-Id: I6bd8e721aa5e3dab53ef28cfdc8fde33eb74ef76

git_command.py

@@ -168,6 +168,9 @@ class GitCommand(object):
       if p is not None:
         s = p + ' ' + s
       _setenv(env, 'GIT_CONFIG_PARAMETERS', s)
+    if 'GIT_ALLOW_PROTOCOL' not in env:
+      _setenv(env, 'GIT_ALLOW_PROTOCOL',
+              'file:git:http:https:ssh:persistent-http:persistent-https:sso')
 
     if project:
       if not cwd: