If a target's .do file disappears, don't forget to stamp it.

If a file previously was generated but now isn't (ie. its .do file
disappears), we would never re-stamp that target, and so all its
dependencies would rebuild continually.

t/deps/gentest.do

@@ -0,0 +1,20 @@
+rm -f genfile2 genfile2.do genfile.log
+
+echo echo hello >genfile2.do
+. ../flush-cache.sh
+redo genfile1
+
+# this will cause a rebuild:
+#      genfile1 depends on genfile2 depends on genfile2.do
+rm -f genfile2.do
+. ../flush-cache.sh
+redo-ifchange genfile1
+
+# but genfile2.do was gone last time, so genfile2 no longer depends on it.
+# thus, it can be considered up-to-date.  Prior versions of redo had a bug
+# where the dependency on genfile2.do was never dropped.
+. ../flush-cache.sh
+redo-ifchange genfile1
+
+COUNT=$(wc -l <genfile.log)
+[ "$COUNT" = 2 ] || exit 77