command-list: prepare machinery for upcoming "common groups" section

The ultimate goal is for "git help" to classify common commands by
group. Toward this end, a subsequent patch will add a new "common
groups" section to command-list.txt preceding the actual command list.
As preparation, teach existing command-list.txt parsing machinery, which
doesn't care about grouping, to skip over this upcoming "common groups"
section.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Sébastien Guimmara <sebastien.guimmara@gmail.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/cmd-list.perl

@@ -38,6 +38,10 @@ sub format_one {
 	}
 }
 
+while (<>) {
+	last if /^### command list/;
+}
+
 my %cmds = ();
 for (sort <>) {
 	next if /^#/;