merge-recursive: do not rudely die on binary merge

merge-recursive: do not rudely die on binary merge

When you try to merge a path that involves binary file-level
merge, merge-recursive died rudely without cleaning up its own
mess.  A files added by the merge were left in the working tree,
but the index was not written out (because it just punted and
died), so it was cumbersome for the user to retry it by first
running "git reset --hard".

This changes merge-recursive to still warn but do the "binary"
merge for such a path; leave the "our" version in the working
tree, but still keep the path unmerged so that the user can sort
it out.

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