In upcoming patches where we try to implement requirements in store, we will
need a mechanism to split all requirements on some basis and decide which one
goes to .hg/requires and which one goes to .hg/store/requires.
This patch introduce a separate function for that. Filtering logic for now is
put under an if False:. In upcoming patches it will be removed.
Since the presence or lack thereof of the requires file is important, I think we should change this to if x is not None so an empty file is written out if appropriate. This should happen, of course. But semantically it is the more correct behavior.