narrow: move requirement constant to core My short-term goal is to move narrowrepo.narrowmatch() onto localrepo and this is a necessary step for that. I put the constant in changegroup.py, unlike REVLOGV2_REQUIREMENT, which is in localrepo.py, since we'll need to access it from the changegroup module eventually. Differential Revision: https://phab.mercurial-scm.org/D2487