Add missing comment in all Makefiles

The comment for the rule diff_alt_orig was missing. It was added like:

    sgit -e '/diff_alt_orig:/i \
    # diff alt and orig' '*Makefile'

2015/Makefile

@@ -184,6 +184,7 @@ diff_orig_alt:
 	done
 	@echo '=-=-= ${YEAR} complete make $@ =-=-='
 
+# diff alt and orig
 diff_alt_orig:
 	@echo '=-=-= ${YEAR} begin make $@ =-=-='
 	@echo