fast-export: Fix output order of D/F changes

fast-export: Fix output order of D/F changes

The fast-import stream format requires incremental changes which take place
immediately, meaning that for D->F conversions all files below the relevant
directory must be deleted before the resulting file of the same name is
created.  Reversing the order can result in fast-import silently deleting
the file right after creating it, resulting in the file missing from the
resulting repository.

We correct this by first sorting the diff_queue_struct in depth-first
order.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>