If the argument to make_file can act like a file, return it. This is useful for code that calls command functions, but doesn't want to send I/O through the filesystem.

#!/bin/sh

set -x
mkdir t
cd t
hg init
echo a > a
hg add a
hg commit -t "test" -u test -d "0 0"
hg verify
hg parents
hg status
hg undo
hg verify
hg parents
hg status