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
+ echo This is file b1
+ hg add b
+ hg commit -t 'commit #1' -d '0 0' -u user
+ hg update 0
+ echo This is file c1
+ hg add c
+ hg commit -t 'commit #2' -d '0 0' -u user
+ hg update -m 1
+ rm b
+ echo This is file c22
+ hg commit -t 'commit #3' -d '0 0' -u user