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 -ex
umask 027
mkdir test1
cd test1
hg init
touch a b
hg add a b
hg ci -t "added a b" -u test -d "0 0"
cd ..
mkdir test2
cd test2
hg init
hg pull ../test1
hg co
chmod +x a
hg ci -t "chmod +x a" -u test -d "0 0"
cd ../test1
echo 123 >>a
hg ci -t "a updated" -u test -d "0 0"
hg pull ../test2
hg heads
hg history
hg -v co -m
ls -l ../test[12]/a > foo
cut -b 0-10 < foo