diff -c/--cc: do not mistake "resolved as deletion" as "use working tree"

diff -c/--cc: do not mistake "resolved as deletion" as "use working tree"

The combined diff machinery can be used to compare:

 - a merge commit with its parent commits;
 - a working-tree file with multiple stages in an unmerged index; or
 - a working-tree file with the HEAD and the index.

The internal function combine-diff.c:show_patch_diff() checked if it needs
to read the "result" from the working tree by looking at the object name
of the result --- if it is null_sha1, it read from the working tree.

This mistook a merge that records a deletion as the conflict resolution
as if it is a cue to read from the working tree. Pass this information
explicitly from the caller instead.

Noticed and reported by Johan Herland.

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