grep: add config knob to enable/disable the default wdir search This allows us to conditionally enable the new behavior to unbreak python-hglib. See the next patch. The config option is undocumented since the --all-files is still experimental and isn't fully implemented. It can be moved to [experimental] if that's preferred. Differential Revision: https://phab.mercurial-scm.org/D3918

--- a/mercurial/configitems.py	Wed Jul 11 21:35:31 2018 +0900
+++ b/mercurial/configitems.py	Wed Jul 11 21:51:47 2018 +0900
@@ -187,6 +187,9 @@
 coreconfigitem('color', 'pagermode',
     default=dynamicdefault,
 )
+coreconfigitem('commands', 'grep.all-files',
+    default=True,
+)
 coreconfigitem('commands', 'show.aliasprefix',
     default=list,
 )