scmutil: introduce filterrequirements() to split reqs into wc and store ones
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.
Differential Revision: https://phab.mercurial-scm.org/D8913