Add merge style fast-forward-only (#28954)

With this option, it is possible to require a linear commit history with
the following benefits over the next best option `Rebase+fast-forward`:
The original commits continue existing, with the original signatures
continuing to stay valid instead of being rewritten, there is no merge
commit, and reverting commits becomes easier.

Closes #24906

tests/integration/pull_merge_test.go

@@ -365,6 +365,90 @@ func TestCantMergeUnrelated(t *testing.T) {
 	})
 }
 
+func TestFastForwardOnlyMerge(t *testing.T) {
+	onGiteaRun(t, func(t *testing.T, giteaURL *url.URL) {
+		session := loginUser(t, "user1")
+		testRepoFork(t, session, "user2", "repo1", "user1", "repo1")
+		testEditFileToNewBranch(t, session, "user1", "repo1", "master", "update", "README.md", "Hello, World 2\n")
+
+		// Use API to create a pr from update to master
+		token := getTokenForLoggedInUser(t, session, auth_model.AccessTokenScopeWriteRepository)
+		req := NewRequestWithJSON(t, http.MethodPost, fmt.Sprintf("/api/v1/repos/%s/%s/pulls", "user1", "repo1"), &api.CreatePullRequestOption{
+			Head:  "update",
+			Base:  "master",
+			Title: "create a pr that can be fast-forward-only merged",
+		}).AddTokenAuth(token)
+		session.MakeRequest(t, req, http.StatusCreated)
+
+		user1 := unittest.AssertExistsAndLoadBean(t, &user_model.User{
+			Name: "user1",
+		})
+		repo1 := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{
+			OwnerID: user1.ID,
+			Name:    "repo1",
+		})
+
+		pr := unittest.AssertExistsAndLoadBean(t, &issues_model.PullRequest{
+			HeadRepoID: repo1.ID,
+			BaseRepoID: repo1.ID,
+			HeadBranch: "update",
+			BaseBranch: "master",
+		})
+
+		gitRepo, err := git.OpenRepository(git.DefaultContext, repo_model.RepoPath(user1.Name, repo1.Name))
+		assert.NoError(t, err)
+
+		err = pull.Merge(context.Background(), pr, user1, gitRepo, repo_model.MergeStyleFastForwardOnly, "", "FAST-FORWARD-ONLY", false)
+
+		assert.NoError(t, err)
+
+		gitRepo.Close()
+	})
+}
+
+func TestCantFastForwardOnlyMergeDiverging(t *testing.T) {
+	onGiteaRun(t, func(t *testing.T, giteaURL *url.URL) {
+		session := loginUser(t, "user1")
+		testRepoFork(t, session, "user2", "repo1", "user1", "repo1")
+		testEditFileToNewBranch(t, session, "user1", "repo1", "master", "diverging", "README.md", "Hello, World diverged\n")
+		testEditFile(t, session, "user1", "repo1", "master", "README.md", "Hello, World 2\n")
+
+		// Use API to create a pr from diverging to update
+		token := getTokenForLoggedInUser(t, session, auth_model.AccessTokenScopeWriteRepository)
+		req := NewRequestWithJSON(t, http.MethodPost, fmt.Sprintf("/api/v1/repos/%s/%s/pulls", "user1", "repo1"), &api.CreatePullRequestOption{
+			Head:  "diverging",
+			Base:  "master",
+			Title: "create a pr from a diverging branch",
+		}).AddTokenAuth(token)
+		session.MakeRequest(t, req, http.StatusCreated)
+
+		user1 := unittest.AssertExistsAndLoadBean(t, &user_model.User{
+			Name: "user1",
+		})
+		repo1 := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{
+			OwnerID: user1.ID,
+			Name:    "repo1",
+		})
+
+		pr := unittest.AssertExistsAndLoadBean(t, &issues_model.PullRequest{
+			HeadRepoID: repo1.ID,
+			BaseRepoID: repo1.ID,
+			HeadBranch: "diverging",
+			BaseBranch: "master",
+		})
+
+		gitRepo, err := git.OpenRepository(git.DefaultContext, repo_model.RepoPath(user1.Name, repo1.Name))
+		assert.NoError(t, err)
+
+		err = pull.Merge(context.Background(), pr, user1, gitRepo, repo_model.MergeStyleFastForwardOnly, "", "DIVERGING", false)
+
+		assert.Error(t, err, "Merge should return an error due to being for a diverging branch")
+		assert.True(t, models.IsErrMergeDivergingFastForwardOnly(err), "Merge error is not a diverging fast-forward-only error")
+
+		gitRepo.Close()
+	})
+}
+
 func TestConflictChecking(t *testing.T) {
 	onGiteaRun(t, func(t *testing.T, giteaURL *url.URL) {
 		user := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})