demilestone should not include milestone (go-gitea#32923)

Fix go-gitea#32887

(cherry picked from commit f44712f)

Conflicts:
	services/issue/milestone_test.go
  trivial conflicts (require vs assert)
(cherry picked from commit 2ffa9a5)

Author: lunny

services/issue/milestone.go

@@ -85,6 +85,10 @@ func changeMilestoneAssign(ctx context.Context, doer *user_model.User, issue *is
 		}
 	}
 
+	if issue.MilestoneID == 0 {
+		issue.Milestone = nil
+	}
+
 	return nil
 }
 

services/issue/milestone_test.go

@@ -24,6 +24,7 @@ func TestChangeMilestoneAssign(t *testing.T) {
 
 	oldMilestoneID := issue.MilestoneID
 	issue.MilestoneID = 2
+	require.NoError(t, issue.LoadMilestone(db.DefaultContext))
 	require.NoError(t, ChangeMilestoneAssign(db.DefaultContext, issue, doer, oldMilestoneID))
 	unittest.AssertExistsAndLoadBean(t, &issues_model.Comment{
 		IssueID:        issue.ID,
@@ -32,4 +33,11 @@ func TestChangeMilestoneAssign(t *testing.T) {
 		OldMilestoneID: oldMilestoneID,
 	})
 	unittest.CheckConsistencyFor(t, &issues_model.Milestone{}, &issues_model.Issue{})
+	assert.NotNil(t, issue.Milestone)
+
+	oldMilestoneID = issue.MilestoneID
+	issue.MilestoneID = 0
+	require.NoError(t, ChangeMilestoneAssign(db.DefaultContext, issue, doer, oldMilestoneID))
+	assert.EqualValues(t, 0, issue.MilestoneID)
+	assert.Nil(t, issue.Milestone)
 }