If a user manually changes a generated file, don't ever overwrite it.

That way the user can modify an auto-generated 'compile' script, for
example, and it'll stay modified.

If they delete the file, we can then generate it for them again.

Also, we have to warn whenever we're doing this, or people might think it's
a bug.

t/flush-cache.sh

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