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>

merge-ort.c

@@ -2193,6 +2193,8 @@ static int handle_content_merge(struct merge_options *opt,
 		clean = merge_submodule(opt, pathnames[0],
 					two_way ? null_oid() : &o->oid,
 					&a->oid, &b->oid, &result->oid);
+		if (clean < 0)
+			return -1;
 		if (opt->priv->call_depth && two_way && !clean) {
 			result->mode = o->mode;
 			oidcpy(&result->oid, &o->oid);