Prevent backup files to be recognized as valid Entity files

The Doctrine2 generator backup Entity files when it adds code into them. The files are named after the original file with a ~ prepended.

The problem is that these backup files are valid PHP files, and as such, they are loaded by Doctrine2. A PHP fatal error is then fired: "Cannot redeclare class...".

Instead of prepending _{, why not appending ~, which is a well supported convention? (foo.php} vs ~foo.php)

Patch is here: https://github.com/doctrine/doctrine2/pull/35