Don't wipe the timestamp when a target fails to redo.

It's really a separate condition.  And since we're not removing the target
*file* in case of error - we update it atomically, and keeping it is better
than losing it - there's no reason to wipe the timestamp in that case
either.

However, we do need to know that the build failed, so that anybody else
(especially in a parallel build) who looks at that target knows that it
died.  So add a separate flag just for that.

t/flush-cache.sh

@@ -4,5 +4,6 @@
 	echo ".timeout 5000"
 	echo "pragma synchronous = off;"
 	echo "update Files set checked_runid=null, " \
-	     "       changed_runid=changed_runid-1;"
+	     "       changed_runid=changed_runid-1, " \
+	     "       failed_runid=null;"
 ) | sqlite3  "$REDO_BASE/.redo/db.sqlite3"