Recursive bake detection

With the issue of #19, I accidentally ran into an unexpected recursive bake invocation like:

```Makefile
foo:
bake bar
bar:
echo bar
```

And since shell would spawn a new process to run the bake, it is hard to get out of this indefinite loop. CTRL-C would not halt the situation. And pid resource would run out quickly until you luckily get a chance to force-quit all the bake process (like `ps aux | grep bake | awk '{print $2}' | xargs kill -9`).

Start with the Makefile reproduction in the issue and trace how bake launches shell commands, especially nested bake invocations and interrupt handling. Reproduce the recursive loop, then define completion as detecting or safely stopping recursive execution without exhausting processes or requiring manual mass termination.

Written by the indexing model from the issue text.