grep: search working directory files by default if --all-files is specified
The default was -rtip:0 before, but "--all-files -rtip:0" would be the most
useless combination, and it isn't supported yet. Let's change the default to
This isn't flagged as BC since --all-files isn't released.
Differential Revision: https://phab.mercurial-scm.org/D3917