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 -x
hg init
echo "[hooks]" > .hg/hgrc
echo 'precommit = echo precommit hook' >> .hg/hgrc
echo 'commit = echo commit hook: $NODE' >> .hg/hgrc
echo a > a
hg add a
hg commit -t "test" -u test -d "0 0"