Handle binary data in diffs

Even though we won't usually display it, binary data may show up
in a diff.  We can't decode it to display it, so just replace it
with a placeholder string so we don't crash.

Change-Id: I4794e707745ed8315afec00c1981e239ec98ef0e

gertty/gitrepo.py

@@ -544,7 +544,10 @@ class Repo(object):
                     if isinstance(line, six.string_types):
                         f.addContextLine(line)
                     else:
-                        f.addContextLine(line.decode('utf8'))
+                        try:
+                            f.addContextLine(line.decode('utf8'))
+                        except:
+                            f.addContextLine("<binary data>")
             f.finalize()
         return files