Target considered up-to-date when prerequisite is newer

This test case:

```sh
set -e

mk="$@"

cat <<EOF > Makefile
a: b
	echo a > a
b: c
	echo b > b
c:
	echo c > c
EOF

${mk}
sleep 0.1
touch c
${mk}
```

fails, because kati does not execute the rule for `a` on the second invocation.