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.
+ hg init+ echo This is file a1+ hg add a+ hg commit -t 'commit #0' -d '0 0' -u user+ lsa+ echo This is file b1+ hg add b+ hg commit -t 'commit #1' -d '0 0' -u user+ hg co 0+ lsa