The rename/copy detection logic in earlier round was only good
enough to show patch output and discussion on the mailing list
about the diff-raw format updates revealed many problems with
it.  This patch fixes all the ones known to me, without making
things I want to do later impossible, mostly related to patch
reordering.

 (1) Earlier rename/copy detector determined which one is rename
     and which one is copy too early, which made it impossible
     to later introduce diffcore transformers to reorder
     patches.  This patch fixes it by moving that logic to the
     very end of the processing.

 (2) Earlier output routine diff_flush() was pruning all the
     "no-change" entries indiscriminatingly.  This was done due
     to my false assumption that one of the requirements in the
     diff-raw output was not to show such an entry (which
     resulted in my incorrect comment about "diff-helper never
     being able to be equivalent to built-in diff driver").  My
     special thanks go to Linus for correcting me about this.
     When we produce diff-raw output, for the downstream to be
     able to tell renames from copies, sometimes it _is_
     necessary to output "no-change" entries, and this patch
     adds diffcore_prune() function for doing it.

 (3) Earlier diff_filepair structure was trying to be not too
     specific about rename/copy operations, but the purpose of
     the structure was to record one or two paths, which _was_
     indeed about rename/copy.  This patch discards xfrm_msg
     field which was trying to be generic for this wrong reason,
     and introduces a couple of fields (rename_score and
     rename_rank) that are explicitly specific to rename/copy
     logic.  One thing to note is that the information in a
     single diff_filepair structure _still_ does not distinguish
     renames from copies, and it is deliberately so.  This is to
     allow patches to be reordered in later stages.

 (4) This patch also adds some tests about diff-raw format
     output and makes sure that necessary "no-change" entries
     appear on the output.

Signed-off-by: Junio C Hamano <junkio@cox.net>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>