Automatic assignment: check for assignment change only

For 'auto_set_status_to_assigned', only change status when bug change
exclusively affects assignment.

A new assignement that is part of a status change is not applied now.
This is the behaviour from 1.2.x

Fixes #20076

Signed-off-by: Damien Regad <dregad@mantisbt.org>

bug_update.php

@@ -365,6 +365,7 @@
 # Handle automatic assignment of issues.
 if( $t_existing_bug->handler_id === NO_USER &&
 	 $t_updated_bug->handler_id !== NO_USER &&
+	 $t_updated_bug->status == $t_existing_bug->status &&
 	 $t_updated_bug->status < config_get( 'bug_assigned_status' ) &&
 	 config_get( 'auto_set_status_to_assigned' ) ) {
 	$t_updated_bug->status = config_get( 'bug_assigned_status' );