Teach git-grep --name-only as synonym for -l

Teach git-grep --name-only as synonym for -l

I expected git grep --name-only to give me only the file names,
much as git diff --name-only only generates filenames.  Alas the
option is -l, which matches common external greps but doesn't match
other parts of the git UI.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>