Always commit if we upgrade 2.x view files

When we upgrade empty view files from 2.x, we end up skipping the commit.
Subsequently, we will see a "wrong signature" error when the view is opened
later. The error is benign as we'd end up resetting an empty view, but it may
surprise an operator. To avoid this, ensure to always commit after upgrading
old views.

Issue: #4994

src/couch_mrview/src/couch_mrview_index.erl

@@ -127,6 +127,7 @@ open(Db, State0) ->
                 {ok, {OldSig, Header}} ->
                     % Matching view signatures.
                     NewSt = init_and_upgrade_state(Db, Fd, State, Header),
+                    ok = commit(NewSt),
                     ensure_local_purge_doc(Db, NewSt),
                     {ok, NewSt};
                 % end of upgrade code for <= 2.x