Fixes state pollution in tup.include'd tupfiles, was preventing error ha...

This pops file in tup.include before descending into the other parsing functions, since it was causing other stack values to be offset later.

The test was:
a.lua:
```lua
x()
```

Tupfile.lua:
```lua
tup.include 'a.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 Tupfile.lua:
[string "Tupfile.lua"]:1: Failed to include file "a.lua".
stack traceback:
	[C]: in function 'include'
	[string "Tupfile.lua"]:1: in main chunk
```

a.lua produces an error either way, but it shouldn't say "error in error handling"