Merge branch 'rs/diff-index-find-copies-harder-optim'

Halve the memory consumed by artificial filepairs created during
"git diff --find-copioes-harder", also making the operation run
faster.

* rs/diff-index-find-copies-harder-optim:
  diff-index: don't queue unchanged filepairs with diff_change()

diff --cc diff.c
Simple merge

diff --cc diff.h
Simple merge