Summary:
This is a bit of a tricky issue; here's the bug in python 2.7
that prevents using the function-like syntax:
https://bugs.python.org/issue21591

The nature of this bug is that scoped exec only works with nested functions
when using the `exec` statement in 2.7.   That is a syntax error in python 3,
which only allows using the `exec` function.

To make the function syntax work in 2.7 we therefore need to avoid using
lambdas or nested functions and have to resort to using a global variable
to act as the "closure".

Refs: https://github.com/facebook/watchman/pull/639

Reviewed By: snarkmaster

Differential Revision: D9552413

fbshipit-source-id: a2cb9d325e18fb6fb06b9e284a84f0a0c80cfe7e