grep: restore pre-9ef10437bb88 behavior, enable wdir search by tweakdefaults

Authored by yuja.

Description

grep: restore pre-9ef10437bb88 behavior, enable wdir search by tweakdefaults

Unfortunately, python-hglib relies on the original grep behavior and is
documented as such. Even though we agreed to introduce the BC, we shouldn't
break existing libraries.

So this patch flips the default again and move the new default to
ui.tweakdefaults. We could instead use HGPLAIN to turn this flag off, but
that would be rather confusing as the old/new behaviors are quite different.

Differential Revision: https://phab.mercurial-scm.org/D3919