Allow multiple projects per issue and pull requests (#36784)

Add ability to add and remove multiple projects per issue
and pull request.

Resolve #12974

---------

Signed-off-by: Icy Avocado <avocado@ovacoda.com>
Co-authored-by: Tyrone Yeh <siryeh@gmail.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: OpenCode (gpt-5.2-codex) <opencode@openai.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

modules/util/diff_slice_test.go

@@ -0,0 +1,74 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package util
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestDiffSliceBasic(t *testing.T) {
+	// Typical integer cases
+	t.Run("additions", func(t *testing.T) {
+		added, removed := DiffSlice([]int{1, 2}, []int{1, 2, 3})
+		assert.Equal(t, []int{3}, added)
+		assert.Empty(t, removed)
+	})
+
+	t.Run("removals", func(t *testing.T) {
+		added, removed := DiffSlice([]int{1, 2, 3}, []int{1, 2})
+		assert.Empty(t, added)
+		assert.Equal(t, []int{3}, removed)
+	})
+
+	t.Run("no changes", func(t *testing.T) {
+		added, removed := DiffSlice([]int{1, 2}, []int{1, 2})
+		assert.Empty(t, added)
+		assert.Empty(t, removed)
+	})
+
+	t.Run("empty slices", func(t *testing.T) {
+		added, removed := DiffSlice([]int{}, []int{})
+		assert.Empty(t, added)
+		assert.Empty(t, removed)
+	})
+
+	t.Run("overlapping elements", func(t *testing.T) {
+		added, removed := DiffSlice([]int{1, 2, 4}, []int{2, 3, 4})
+		assert.Equal(t, []int{3}, added)
+		assert.Equal(t, []int{1}, removed)
+	})
+}
+
+func TestDiffSliceOrderAndDuplicates(t *testing.T) {
+	oldSlice := []int{1, 2, 2, 3}
+	newSlice := []int{2, 4, 2, 5}
+
+	added, removed := DiffSlice(oldSlice, newSlice)
+	assert.Equal(t, []int{4, 5}, added)
+	assert.Equal(t, []int{1, 3}, removed)
+}
+
+func TestDiffSliceDeduplicatesOutput(t *testing.T) {
+	// Test case from issue: newSlice contains [4, 4, 5] and oldSlice is [1]
+	// added should return [4, 5], not [4, 4, 5]
+	t.Run("deduplicates added", func(t *testing.T) {
+		added, removed := DiffSlice([]int{1}, []int{4, 4, 5})
+		assert.Equal(t, []int{4, 5}, added)
+		assert.Equal(t, []int{1}, removed)
+	})
+
+	t.Run("deduplicates removed", func(t *testing.T) {
+		added, removed := DiffSlice([]int{1, 1, 2}, []int{3})
+		assert.Equal(t, []int{3}, added)
+		assert.Equal(t, []int{1, 2}, removed)
+	})
+
+	t.Run("deduplicates both", func(t *testing.T) {
+		added, removed := DiffSlice([]int{1, 1, 2, 2}, []int{3, 3, 4, 4})
+		assert.Equal(t, []int{3, 4}, added)
+		assert.Equal(t, []int{1, 2}, removed)
+	})
+}