t/deps/dirtest: test correct dependency checking for targets named 'dirname'

We had a bug (fixed in the previous commit) where doing 'redo-ifchange
dirname' (which runs dirname/all.do) would not create the stamp correctly,
so that it would always show up as dirty.

It's a little bit complicated to simulate, but this does it.

t/deps/dirtest/test.do

@@ -0,0 +1,19 @@
+touch t?.do
+for first in t1 t2 t3; do
+	for second in t1 t2 t3; do
+		rm -f log dir1/log dir1/stinky
+		. ../../flush-cache.sh
+		redo $first
+		touch $second.do
+		. ../../flush-cache.sh
+		redo $second
+		. ../../flush-cache.sh
+		redo-ifchange $second
+		C1="$(wc -l <dir1/log)"
+		C2="$(wc -l <log)"
+		if [ "$C1" != 1 -o "$C2" != 2 ]; then
+			echo "failed: $first>$second, c1=$C1, c2=$C2" >&2
+			exit 55
+		fi
+	done
+done