merge-ort: fix type of local 'clean' var in handle_content_merge ()

merge-ort: fix type of local 'clean' var in handle_content_merge ()

handle_content_merge() returns an int.  Every caller of
handle_content_merge() expects an int.  However, we declare a local
variable 'clean' that we use for the return value to be unsigned.  To
make matters worse, we also assign 'clean' the return value of
merge_submodule() in one codepath, which is defined to return an int.
It seems that the only reason to have 'clean' be unsigned was to allow a
cutesy bit manipulation operation to be well-defined.  Fix the type of
the 'clean' local in handle_content_merge().

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