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
mkdir t
cd t
hg init
echo This is file a1 > a
hg add a
hg commit -t "commit #0" -d "0 0" -u user
echo This is file b1 > b
hg add b
hg commit -t "commit #1" -d "0 0" -u user
rm b
hg update 0
echo This is file b2 > b
hg add b
hg commit -t "commit #2" -d "0 0" -u user
cd ..; /bin/rm -rf t
mkdir t
cd t
hg init
echo This is file a1 > a
hg add a
hg commit -t "commit #0" -d "0 0" -u user
echo This is file b1 > b
hg add b
hg commit -t "commit #1" -d "0 0" -u user
rm b
hg update 0
echo This is file b2 > b
hg commit -A -t "commit #2" -d "0 0" -u user
cd ..; /bin/rm -rf t
mkdir t
cd t
hg init
echo This is file a1 > a
hg add a
hg commit -t "commit #0" -d "0 0" -u user
echo This is file b1 > b
hg add b
hg commit -t "commit #1" -d "0 0" -u user
rm b
hg remove b
hg update 0
echo This is file b2 > b
hg commit -A -t "commit #2" -d "0 0" -u user
cd ..; /bin/rm -rf t