Add missed transaction on setmerged (#33079)

Follow #33045. There are two updates on `Set Merged`, which should be in
one transaction.
This also introduced some refactors for changeissuestatus to make it
more clear.

models/issues/pull.go

@@ -80,22 +80,6 @@ func (err ErrPullRequestAlreadyExists) Unwrap() error {
 	return util.ErrAlreadyExist
 }
 
-// ErrPullWasClosed is used close a closed pull request
-type ErrPullWasClosed struct {
-	ID    int64
-	Index int64
-}
-
-// IsErrPullWasClosed checks if an error is a ErrErrPullWasClosed.
-func IsErrPullWasClosed(err error) bool {
-	_, ok := err.(ErrPullWasClosed)
-	return ok
-}
-
-func (err ErrPullWasClosed) Error() string {
-	return fmt.Sprintf("Pull request [%d] %d was already closed", err.ID, err.Index)
-}
-
 // PullRequestType defines pull request type
 type PullRequestType int