Don't capture stdout during 'repo checkout'

There isn't any great value in buffering stdout into memory
coming from git checkout.  So don't bother doing it.

Signed-off-by: Shawn O. Pearce <sop@google.com>

project.py

@@ -771,7 +771,7 @@ class Project(object):
 
     # Do the checkout
     cmd = ['checkout', name, '--']
-    return GitCommand(self, cmd, capture_stdout=True).Wait() == 0
+    return GitCommand(self, cmd).Wait() == 0
 
   def AbandonBranch(self, name):
     """Destroy a local topic branch.