Bug 1785514: Only update auth instance for this user

When we add a user to an institution that doesn't have internal auth
on the admin/users/edit.php page

behatnotneeded

Change-Id: I7395d1481b15c0e1e82eec0a61b3cca891df72f9
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>

htdocs/admin/users/edit.php

@@ -186,8 +186,8 @@ foreach ($user_insts as $inst) {
             $internal = insert_record('auth_instance', $todb, 'id', true);
         }
         // Now we know we have an internal auth for the user, set it
-        set_field('usr', 'authinstance', $internal);
         $user->authinstance = $internal;
+        $user->commit();
     }
 }