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

--- a/mercurial/hook.py	Mon Dec 14 17:59:10 2020 -0500
+++ b/mercurial/hook.py	Mon Dec 14 18:03:29 2020 -0500
@@ -157,6 +157,7 @@
             env[b'HG_PENDING'] = repo.root
     env[b'HG_HOOKTYPE'] = htype
     env[b'HG_HOOKNAME'] = name
+    env[b'HGPLAIN'] = b'1'
 
     for k, v in pycompat.iteritems(args):
         # transaction changes can accumulate MBs of data, so skip it