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
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