Handle a diff with only an svn property change

cdiff.py

                 self._type = 'unified'
                 break
         else:
-            if size < 4:
+            if size < 5:
                 # It's safe to consider as udiff if patch stream contains no
-                # more than 3 lines, happens with `git diff` on a file that
-                # only has perm bits changes
+                # more than 4 lines. happens with `git diff` on a file that
+                # only has perm bits changes or `svn diff` with property
+                # changes.
                 #
                 self._type = 'unified'
             else:

tests/svn-property/in.diff

+Index: foo
+===================================================================
+Cannot display: file marked as a binary type.
+svn:mime-type = application/octet-stream

tests/svn-property/out.normal

+Index: foo
+===================================================================
+Cannot display: file marked as a binary type.
+svn:mime-type = application/octet-stream
+

tests/svn-property/out.side-by-side

+Index: foo
+===================================================================
+Cannot display: file marked as a binary type.
+svn:mime-type = application/octet-stream
+

tests/svn-property/out.w70

+Index: foo
+===================================================================
+Cannot display: file marked as a binary type.
+svn:mime-type = application/octet-stream
+

tests/test_cdiff.py

 --- a
 spam
 +++ b
+
 """
         items = patch.splitlines(True)
         stream = cdiff.PatchStream(Sequential(items))