framework: add git-hook to check PORTEPOCH validity

• checks dropping of PORTEPOCH:

[pre-commit] dropped PORTEPOCH 1 in net/kf5-kdav/Makefile

• checks it being non-decreasing:

[pre-commit] PORTEPOCH decreasing from 2 to 1 in net-p2p/frost/Makefile

Reviewed By: rene, bapt
Differential Revision: https://reviews.freebsd.org/D35733