hooks: add a `auto` value for `hooks.*run-with-plain`

That setting restore the behavior pre-5.6. The current HGPLAIN value is simply
passed to the hooks.

This allow user who needs it to fully mitigate the behavior change introduced
in Mercurial 5.7 by restoring the older behavior.

Differential Revision: https://phab.mercurial-scm.org/D9982

mercurial/helptext/config.txt

@@ -1032,6 +1032,8 b' Example ``.hg/hgrc``::'
   incoming.autobuild:run-with-plain = yes
   # HGPLAIN never set
   incoming.autobuild:run-with-plain = no
+  # HGPLAIN inherited from environment (default before Mercurila 5.7)
+  incoming.autobuild:run-with-plain = auto
 
 Most hooks are run with environment variables set that give useful
 additional information. For each hook below, the environment variables

mercurial/hook.py

@@ -158,7 +158,10 b' def _exthook(ui, repo, htype, name, cmd,'
     env[b'HG_HOOKTYPE'] = htype
     env[b'HG_HOOKNAME'] = name
 
-    plain = ui.configbool(b'hooks', b'%s:run-with-plain' % name)
+    if ui.config(b'hooks', b'%s:run-with-plain' % name) == b'auto':
+        plain = ui.plain()
+    else:
+        plain = ui.configbool(b'hooks', b'%s:run-with-plain' % name)
     if plain:
         env[b'HGPLAIN'] = b'1'
     else:

tests/test-hook.t

@@ -1412,16 +1412,20 b' HGPLAIN setting in hooks'
   > pre-version.testing-yes:run-with-plain=yes
   > pre-version.testing-no=echo '### no ########' plain: \${HGPLAIN:-'<unset>'}
   > pre-version.testing-no:run-with-plain=no
+  > pre-version.testing-auto=echo '### auto ######' plain: \${HGPLAIN:-'<unset>'}
+  > pre-version.testing-auto:run-with-plain=auto
   > EOF
 
   $ (unset HGPLAIN; hg version --quiet)
   ### default ### plain: 1
   ### yes ####### plain: 1
   ### no ######## plain: <unset>
+  ### auto ###### plain: <unset>
   Mercurial Distributed SCM (*) (glob)
 
   $ HGPLAIN=1 hg version --quiet
   ### default ### plain: 1
   ### yes ####### plain: 1
   ### no ######## plain: <unset>
+  ### auto ###### plain: 1
   Mercurial Distributed SCM (*) (glob)