grep: run setup_git_directory_gently() sooner

grep: run setup_git_directory_gently() sooner

git grep already runs a repository search unconditionally,
even when the --no-index option is supplied; running such a
search earlier is not very risky.

Just like with shortlog, without this change, the
“[pager] grep” configuration is not respected at all.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>