merge-recursive: Improve handling of rename target vs. directory addition

merge-recursive: Improve handling of rename target vs. directory addition

When dealing with file merging and renames and D/F conflicts and possible
criss-cross merges (how's that for a corner case?), we did not do a
thorough job ensuring the index and working directory had the correct
contents.   Fix the logic in merge_content() to handle this.  Also,
correct some erroneous tests in t6022 that were expecting the wrong number
of unmerged index entries.  These changes fix one of the tests in t6042
(and almost fix another one from t6042 as well).

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>