py3: fix slicing of byte string in revlog.compress()

I tried .startswith('\0'), but data wasn't always a bytes nor a bytearray.

mercurial/revlog.py

@@ -1488,7 +1488,7 b' class revlog(object):'
             # The revlog compressor added the header in the returned data.
             return '', compressed
 
-        if data[0] == '\0':
+        if data[0:1] == '\0':
             return '', data
         return 'u', data