merge-ort: clearer propagation of failure-to-function from merge_submodule

merge-ort: clearer propagation of failure-to-function from merge_submodule

The 'clean' member variable is somewhat of a tri-state (1 = clean, 0 =
conflicted, -1 = failure-to-determine), but we often like to think of
it as binary (ignoring the possibility of a negative value) and use
constructs like '!clean' to reflect this.  However, these constructs
can make codepaths more difficult to understand, unless we handle the
negative case early and return pre-emptively; do that in
handle_content_merge() to make the code a bit easier to read.

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