generate-cmdlist.sh: run "grep | sort", not "sort | grep"

This doesn't matter for performance, but let's not include the empty
lines in our sorting. This makes the intent of the code clearer.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

generate-cmdlist.sh

@@ -18,8 +18,8 @@ category_list () {
 	command_list "$1" |
 	cut -c 40- |
 	tr ' ' '\012' |
-	LC_ALL=C sort -u |
-	grep -v '^$'
+	grep -v '^$' |
+	LC_ALL=C sort -u
 }
 
 get_synopsis () {