Switch description and message reporting in `clar__assert_equal_file`

`clar__fail` will dup the description, but not the error message, on the assumption that the description was something that you crafted as a nice message but the error message is a string constant that will never get freed.  `clar__assert_equal_file` should behave with that assumption in mind.

Before:

    1) Failure:
    rebase::merge::first_next [..\tests\rebase\merge.c:36]
      4á╧☺`^É½♥
      rebase/.git/rebase-merge/current

After:

    1) Failure:
    rebase::merge::first_next [..\tests\rebase\merge.c:36]
      rebase/.git/rebase-merge/current
      file content mismatch at byte 0