Should not recursively chown all files in the host execution directory

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Use an account whose UID or GID is different from AppPot's defaults
2. Call `apppot-start.sh` in a directory with many files
3. Watch the startup sequence stall when it comes to the message "Setting up 
AppPot user user(6909) in group users(4200)"

What is the expected output? What do you see instead?

It should only change ownership of files within the AppPot filesystem;
changing ownership in the host filesystem is an expensive no-op.

Original issue reported on code.google.com by riccardo.murri@gmail.com on 8 Feb 2012 at 7:40