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
Differential Revision: https://phab.mercurial-scm.org/D3918