[RFC] allow fulltext to be more dynamic

[Henri Wiechers]

Looks good.

One thing though, it assumes that the only two methods are 'fulltext'
and 'line-delta'. It may not work if new methods are introduced. Maybe
it would be better to do something like:

while count < self._max_delta_chain:
...
else:
    return False

return fulltext_size >= delta_size

Henri