Pass "." to git grep to satisfy "all paths" for Git 2.16.0

Summary:
Ref T13050. See https://discourse.phabricator-community.org/t/issues-with-git-2-16-0/1004/2.

secure picked up 2.16.0 so this reproduces now: https://secure.phabricator.com/source/phabricator/browse/master/?grep=dog

Test Plan: Will push.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13050

Differential Revision: https://secure.phabricator.com/D18944