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 a > a
hg add a
hg commit -t "test" -u test -d "0 0"
hg history
hg tag -u test -d "0 0" "bleah"
hg history
echo foo >> .hgtags
hg tag -u test -d "0 0" "bleah2" || echo "failed"