This introduces the diff-core, the layer between the diff-tree
family and the external diff interface engine.  The calls to the
interface diff-tree family uses (diff_change and diff_addremove)
have not changed and will not change.  The purpose of the
diff-core layer is to provide an infrastructure to transform the
set of differences sent from the applications, before sending
them to the external diff interface.

The recently introduced rename detection code has been rewritten
to use the diff-core facility.  When applications send in
separate creates and deletes, matching ones are transformed into
a single rename-and-edit diff, and sent out to the external diff
interface as such.

This patch also enhances the rename detection code further to be
able to detect copies.  Currently this happens only as long as
copy sources appear as part of the modified files, but there
already is enough provision for callers to report unmodified
files to diff-core, so that they can be also used as copy source
candidates.  Extending the callers this way will be done in a
separate patch.

Please see and marvel at how well this works by trying out the
newly added t/t4003-diff-rename-1.sh test script.

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