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

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

--- a/mercurial/helptext/config.txt	Wed Feb 10 23:21:21 2021 +0100
+++ b/mercurial/helptext/config.txt	Wed Feb 10 23:03:54 2021 +0100
@@ -1032,6 +1032,8 @@
   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

--- a/mercurial/hook.py	Wed Feb 10 23:21:21 2021 +0100
+++ b/mercurial/hook.py	Wed Feb 10 23:03:54 2021 +0100
@@ -158,7 +158,10 @@
     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:

--- a/tests/test-hook.t	Wed Feb 10 23:21:21 2021 +0100
+++ b/tests/test-hook.t	Wed Feb 10 23:03:54 2021 +0100
@@ -1412,16 +1412,20 @@
   > 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)