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

mercurial/hook.py

@@ -157,6 +157,7 b' def _exthook(ui, repo, htype, name, cmd,'
             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