rebase -i: recognize short commands without arguments

The sequencer instruction 'b', short for 'break', is rejected:

  error: invalid line 2: b

The reason is that the parser expects all short commands to have
an argument. Permit short commands without arguments.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-rebase.sh

@@ -49,7 +49,7 @@ set_fake_editor () {
 		case $line in
 		squash|fixup|edit|reword|drop)
 			action="$line";;
-		exec*|break)
+		exec*|break|b)
 			echo "$line" | sed 's/_/ /g' >> "$1";;
 		"#")
 			echo '# comment' >> "$1";;