This rips out the rename detection engine from diff-helper and moves it
to the diff core, and updates the internal calling convention used by
diff-tree family into the diff core.  In order to give the same option
name to diff-tree family as well as to diff-helper, I've changed the
earlier diff-helper '-r' option to '-M' (stands for Move; sorry but the
natural abbreviation 'r' for 'rename' is already taken for 'recursive').

Although I did a fair amount of test with the git-diff-tree with
existing rename commits in the core GIT repository, this should still be
considered beta (preview) release.  This patch depends on the diff-delta
infrastructure just committed.

This implements almost everything I wanted to see in this series of
patch, except a few minor cleanups in the calling convention into diff
core, but that will be a separate cleanup patch.
Signed-off-by: NJunio C Hamano <junkio@cox.net>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>