git: un-byteify the `mode` argument for the builtin `open()`

I guess this was assuming `pycompat.open` was imported, but it's not here or
elsewhere in the git extension.

Differential Revision: https://phab.mercurial-scm.org/D12568

hgext/git/dirstate.py

@@ -33,7 +33,7 b' def readpatternfile(orig, filepath, warn'
         return orig(filepath, warn, sourceinfo=False)
     result = []
     warnings = []
-    with open(filepath, b'rb') as fp:
+    with open(filepath, 'rb') as fp:
         for l in fp:
             l = l.strip()
             if not l or l.startswith(b'#'):