Fix concurrent stack modification

This uses the correct stack to prevent concurrent stack modification when recursively loading files.  The example is a little contrived - it mirrors a situation with my own Tupfiles... There's probably something more illustrative but I'm not sure I fully understand the consequences of the bug.

The test was:
a.lua:
```lua
x()
```

Tuprules.lua:
```lua
tup.include 'a.lua'
```

Tupfile.lua:
```lua
```

It printed:
```
tup error: Failed to execute a.lua:
error in error handling
tup error: Failed to parse included file 'a.lua'
tup error: Failed to execute Tuprules.lua:
[string "Tuprules.lua"]:1: Failed to include file "a.lua".
stack traceback:
	[C]: in function 'include'
	[string "Tuprules.lua"]:1: in main chunk
tup error: Failed to parse included file 'Tuprules.lua'
```

a.lua produces an error either way, but it shouldn't say "error in error handling"