merge-ort: avoid surprise with new sub_flag variable

Commit 4057523 ("submodule merge: update conflict error message",
2022-08-04) added a sub_flag variable that is used to store a value from
enum conflict_and_info_types, but initializes it with a value of -1 that
does not correspond to any of the conflict_and_info_types.  The code may
never set it to a valid value and yet still use it, which can be
surprising when reading over the code at first.  Initialize it instead
to the generic CONFLICT_SUBMODULE_FAILED_TO_MERGE value, which is still
distinct from the two values we need to special case.

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

merge-ort.c

@@ -1767,7 +1767,7 @@ static int merge_submodule(struct merge_options *opt,
 	int i;
 	int search = !opt->priv->call_depth;
 	int sub_not_initialized = 1;
-	int sub_flag = -1;
+	int sub_flag = CONFLICT_SUBMODULE_FAILED_TO_MERGE;
 
 	/* store fallback answer in result in case we fail */
 	oidcpy(result, opt->priv->call_depth ? o : a);