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 This is file a1 > a
hg add a
hg commit -t "commit #0" -d "0 0" -u user
ls
echo This is file b1 > b
hg add b
hg commit -t "commit #1" -d "0 0" -u user
hg co 0
# B should disappear
ls