reset --hard/read-tree --reset -u: remove unmerged new paths

reset --hard/read-tree --reset -u: remove unmerged new paths

When aborting a failed merge that has brought in a new path using "git
reset --hard" or "git read-tree --reset -u", we used to first forget about
the new path (via read_cache_unmerged) and then matched the working tree
to what is recorded in the index, thus ending up leaving the new path in
the work tree.

Signed-off-by: Junio C Hamano <gitster@pobox.com>