Add a redo-always command: it adds an "always dirty" dependency to your target.

This is mostly useless except when combined with redo-stamp... I think.

t/alwaystest.do

@@ -0,0 +1,13 @@
+rm -f always1 always1.log
+
+redo always1
+[ "$(wc -l <always1.log)" -eq 1 ] || exit 11
+
+# This shouldn't rebuild, but because other people might be running flush-cache.sh
+# in parallel with us, we can't be 100% sure it won't.  So don't test it.
+#redo-ifchange always1
+#[ "$(wc -l <always1.log)" -eq 1 ] || exit 21
+
+./flush-cache.sh
+redo-ifchange always1
+[ "$(wc -l <always1.log)" -eq 2 ] || exit 31