hook: set `HGPLAIN=1` for external hooks...
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
