[FEAT] Check if commit is already present in target branch

- Check if someone is (accidentally) trying to create a pull request via
AGit with changes already in the target branch and fail if that is the
case.
- Added integration test.

services/agit/agit.go

@@ -101,6 +101,21 @@ func ProcReceive(ctx context.Context, repo *repo_model.Repository, gitRepo *git.
 				return nil, fmt.Errorf("failed to get unmerged AGit flow pull request in repository %q: %w", repo.FullName(), err)
 			}
 
+			// Check if the changes are already in the target branch.
+			stdout, _, gitErr := git.NewCommand(ctx, "branch", "--contains").AddDynamicArguments(opts.NewCommitIDs[i], baseBranchName).RunStdString(&git.RunOpts{Dir: repo.RepoPath()})
+			if gitErr != nil {
+				return nil, fmt.Errorf("failed to check if the target branch already contains the new commit in repository %q: %w", repo.FullName(), err)
+			}
+			if len(stdout) > 0 {
+				results = append(results, private.HookProcReceiveRefResult{
+					OriginalRef: opts.RefFullNames[i],
+					OldOID:      opts.OldCommitIDs[i],
+					NewOID:      opts.NewCommitIDs[i],
+					Err:         "The target branch already contains this commit",
+				})
+				continue
+			}
+
 			// Automatically fill out the title and the description from the first commit.
 			shouldGetCommit := len(title) == 0 || len(description) == 0
 

tests/integration/git_test.go

@@ -937,13 +937,13 @@ func doCreateAgitFlowPull(dstPath string, ctx *APITestContext, baseBranch, headB
 			})
 		})
 
+		upstreamGitRepo, err := git.OpenRepository(git.DefaultContext, filepath.Join(setting.RepoRootPath, ctx.Username, ctx.Reponame+".git"))
+		require.NoError(t, err)
+		defer upstreamGitRepo.Close()
+
 		t.Run("Force push", func(t *testing.T) {
 			defer tests.PrintCurrentTest(t)()
 
-			upstreamGitRepo, err := git.OpenRepository(git.DefaultContext, filepath.Join(setting.RepoRootPath, ctx.Username, ctx.Reponame+".git"))
-			require.NoError(t, err)
-			defer upstreamGitRepo.Close()
-
 			_, _, gitErr := git.NewCommand(git.DefaultContext, "push", "origin").AddDynamicArguments("HEAD:refs/for/master/" + headBranch + "-force-push").RunStdString(&git.RunOpts{Dir: dstPath})
 			require.NoError(t, gitErr)
 
@@ -984,6 +984,21 @@ func doCreateAgitFlowPull(dstPath string, ctx *APITestContext, baseBranch, headB
 			})
 		})
 
+		t.Run("Branch already contains commit", func(t *testing.T) {
+			defer tests.PrintCurrentTest(t)()
+
+			branchCommit, err := upstreamGitRepo.GetBranchCommit("master")
+			require.NoError(t, err)
+
+			_, _, gitErr := git.NewCommand(git.DefaultContext, "reset", "--hard").AddDynamicArguments(branchCommit.ID.String() + "~1").RunStdString(&git.RunOpts{Dir: dstPath})
+			require.NoError(t, gitErr)
+
+			_, stdErr, gitErr := git.NewCommand(git.DefaultContext, "push", "origin").AddDynamicArguments("HEAD:refs/for/master/" + headBranch + "-already-contains").RunStdString(&git.RunOpts{Dir: dstPath})
+			assert.Error(t, gitErr)
+
+			assert.Contains(t, stdErr, "already contains this commit")
+		})
+
 		t.Run("Merge", doAPIMergePullRequest(*ctx, ctx.Username, ctx.Reponame, pr1.Index))
 		t.Run("CheckoutMasterAgain", doGitCheckoutBranch(dstPath, "master"))
 	}