How do I determine the existence (or non-existence) of a file from an AWK script?

[Alan Mackenzie]

Hello, gawk.

This should be something quite simple, but I can't work out how to do it.

Given a file name in an AWK script, does a file with that name actually
exist?  In a shell script, I'd simply do [ -f "$FOO" ].  What do I have
to do in AWK?

The context is a script which checks certain syntactic things in a git
commit message.  I want to extend it to check that the noted file names
in the commit message don't have typos, or missing directory parts, that
kind of thing.

Thanks!

-- 
Alan Mackenzie (Nuremberg, Germany).