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>

Documentation/git-grep.txt

@@ -75,9 +75,11 @@ OPTIONS
 -n::
 	Prefix the line number to matching lines.
 
--l | --files-with-matches | -L | --files-without-match::
+-l | --files-with-matches | --name-only | -L | --files-without-match::
 	Instead of showing every matched line, show only the
 	names of files that contain (or do not contain) matches.
+	For better compatability with git-diff, --name-only is a
+	synonym for --files-with-matches.
 
 -c | --count::
 	Instead of showing every matched line, show the number of