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>
diff --git a/builtin-grep.c b/builtin-grep.c
index 9180b39..f4f4ecb 100644
--- a/builtin-grep.c
+++ b/builtin-grep.c
@@ -578,6 +578,7 @@
 			continue;
 		}
 		if (!strcmp("-l", arg) ||
+		    !strcmp("--name-only", arg) ||
 		    !strcmp("--files-with-matches", arg)) {
 			opt.name_only = 1;
 			continue;