fast-export: ensure that a renamed file is printed after all references

fast-export: ensure that a renamed file is printed after all references

t9350 sets up a commit where a file is both copied and renamed. The output
of fast-export for this commit should look like this:

  author ...
  committer ...
  from :19
  C "file2" "file4"
  R "file2" "file5"

The order of the two modification lines is derived from the result that
the diff machinery produces.

060df62 (fast-export: Fix output order of D/F changes) inserted a qsort
call that modifies the order of the diff result. Unfortunately, qsort need
not be stable. Therefore, it is possible that the 'R' line appears before
the 'C' line and the resulting fast-import stream is incorrect.

Fix it by forcing that the rename entry is printed after all other
modification lines with the same file name.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>