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
hg init
echo a > a
hg add a
hg commit -t "1" -u test -d "0 0"
hg status
cp a b
hg copy a b
hg status
hg --debug commit -t "2" -u test -d "0 0"
hg history
hg log a
hexdump -C .hg/data/b.d
hg cat b > bsum
md5sum bsum
hg cat a > asum
md5sum asum
hg verify