Mach bootstrapping will sync modules

Fixes #4916

python/servo/command_base.py

@@ -166,6 +166,7 @@ class CommandBase(object):
         if self.context.bootstrapped:
             return
 
+        subprocess.check_call(["git", "submodule", "sync"])
         submodules = subprocess.check_output(["git", "submodule", "status"])
         for line in submodules.split('\n'):
             components = line.strip().split(' ')