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
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a