t/curse: test to make sure the same file isn't generated more than once.

redo currently passes this when running serially, but not in parallel.

t/curse/.gitignore

@@ -1,2 +1,3 @@
 *.n1
 *.n2
+*.count

t/curse/all.do

@@ -1,3 +1,8 @@
 redo-ifchange 1.n0 2.n0 3.n0
 DEPS=$(seq 10 | sed 's/$/.n1/')
 redo-ifchange $DEPS
+COUNT=$(cat *.count | wc -l)
+if ! [ "$COUNT" = 100 ]; then
+	echo "expected 100 writes, got $COUNT" >&2
+	exit 42
+fi

t/curse/clean.do

@@ -1 +1,2 @@
-rm -f *~ .*~ *.n1 *.n2 *.tmp
+rm -f *~ .*~ *.n1 *.n2 *.tmp *.count
+

t/curse/default.n2.do

@@ -1 +1,2 @@
 echo n2-$1
+echo $1 >>$1.count