add_files_to_cache() refreshes the index from worktree changes and does
not need rename detection. When unmerged entries and a deleted stage-0
path are present together, rename detection can pair them and rewrite an
unmerged diff pair to point at the deleted path.
That later makes "git commit -a" and "git add -u" try to stat the
deleted path and die with "unable to stat". Disable rename detection in
this callback-driven staging path and add a regression test covering the
crash.
Signed-off-by: Nick Golden <blindmansion@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>