decode the buffer before appending

output from a process is in bytes in python3. we need
to decode it.

in Python3, bytes don't have an encode attribute. use this
to identify it.

Change-Id: I152f2ec34614131027db680ead98b53f9b321ed5

git_command.py

@@ -263,6 +263,8 @@ class GitCommand(object):
         if not buf:
           s_in.remove(s)
           continue
+        if not hasattr(buf, 'encode'):
+          buf = buf.decode()
         if s.std_name == 'stdout':
           self.stdout += buf
         else: