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.
+ hg init
+ echo '[hooks]'
+ echo 'precommit = echo precommit hook'
+ echo 'commit = echo commit hook: $NODE'
+ echo a
+ hg add a
+ hg commit -t test -u test -d '0 0'
precommit hook
commit hook: acb14030fe0a21b60322c440ad2d20cf7685a376