hook: set `HGPLAIN=1` for external hooks External hooks will probably turn around and run hg commands, so this seems like a reasonable convenience for people who miss it in the documentation. There are no test changes because `printenv.py` filters out anything without a "HG_" prefix. It wouldn't be a useful test anyway, because this is already inherited from the test environment. Differential Revision: https://phab.mercurial-scm.org/D9605