feat: improved performances when checking for conflicts on pull requests (#7727)

- `testPatch` is a function that is called to test a pull request and determine the state of the pull request. Checking for merge conflicts, check if the diff is empty and if the pull request modifies any protected files.
- The checking for merge conflict and if the diff is empty used git commands that relied on a working tree to correctly functions. Forgejo store repositories in a bare format which do not contain a working tree. This means that a temporary copy was created every time a pull request had to be re-checked and for large repositories involving quite some I/O interaction.
- This patch adjusts those codepaths to instead use newer Git plumbing commands that work without requiring a work tree and can thus be used directly on the bare repository. The merge conflict is now done via [`git-merge-tree(1)`](https://git-scm.com/docs/git-merge-tree/) and checking if the diff is empty is done via [`git-diff-tree(1)`](https://git-scm.com/docs/git-diff-tree).
- If the function is called to test a patch where the head and base repository are not the same, then [Git alternate](https://git-scm.com/docs/gitglossary#Documentation/gitglossary.txt-aiddefalternateobjectdatabaseaalternateobjectdatabase) is used to make the head commit available in the base repository, this done on a per git command basis via the `GIT_ALTERNATE_OBJECT_DIRECTORIES` environment.
- As far as I can understand the documentation and the existing code, there's no edge case that the new code cannot handle. It also results in a cleaner codepath, as the existing code did a lot of checking and merging in a more traditional approach that required a lot of (parsing) code, while the new code offloads this to git and has a trivial parser of the output.
- Resolves forgejo/forgejo#7701
- Added exhaustive integration testing.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7727
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Reviewed-by: Otto <otto@codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

tests/integration/pull_merge_test.go

@@ -40,7 +40,6 @@ import (
 	"forgejo.org/services/forms"
 	"forgejo.org/services/pull"
 	commitstatus_service "forgejo.org/services/repository/commitstatus"
-	files_service "forgejo.org/services/repository/files"
 	webhook_service "forgejo.org/services/webhook"
 	"forgejo.org/tests"
 
@@ -517,78 +516,6 @@ func TestCantFastForwardOnlyMergeDiverging(t *testing.T) {
 	})
 }
 
-func TestConflictChecking(t *testing.T) {
-	onGiteaRun(t, func(t *testing.T, giteaURL *url.URL) {
-		user := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})
-
-		// Create new clean repo to test conflict checking.
-		baseRepo, _, f := tests.CreateDeclarativeRepo(t, user, "conflict-checking", nil, nil, nil)
-		defer f()
-
-		// create a commit on new branch.
-		_, err := files_service.ChangeRepoFiles(git.DefaultContext, baseRepo, user, &files_service.ChangeRepoFilesOptions{
-			Files: []*files_service.ChangeRepoFile{
-				{
-					Operation:     "create",
-					TreePath:      "important_file",
-					ContentReader: strings.NewReader("Just a non-important file"),
-				},
-			},
-			Message:   "Add a important file",
-			OldBranch: "main",
-			NewBranch: "important-secrets",
-		})
-		require.NoError(t, err)
-
-		// create a commit on main branch.
-		_, err = files_service.ChangeRepoFiles(git.DefaultContext, baseRepo, user, &files_service.ChangeRepoFilesOptions{
-			Files: []*files_service.ChangeRepoFile{
-				{
-					Operation:     "create",
-					TreePath:      "important_file",
-					ContentReader: strings.NewReader("Not the same content :P"),
-				},
-			},
-			Message:   "Add a important file",
-			OldBranch: "main",
-			NewBranch: "main",
-		})
-		require.NoError(t, err)
-
-		// create Pull to merge the important-secrets branch into main branch.
-		pullIssue := &issues_model.Issue{
-			RepoID:   baseRepo.ID,
-			Title:    "PR with conflict!",
-			PosterID: user.ID,
-			Poster:   user,
-			IsPull:   true,
-		}
-
-		pullRequest := &issues_model.PullRequest{
-			HeadRepoID: baseRepo.ID,
-			BaseRepoID: baseRepo.ID,
-			HeadBranch: "important-secrets",
-			BaseBranch: "main",
-			HeadRepo:   baseRepo,
-			BaseRepo:   baseRepo,
-			Type:       issues_model.PullRequestGitea,
-		}
-		err = pull.NewPullRequest(git.DefaultContext, baseRepo, pullIssue, nil, nil, pullRequest, nil)
-		require.NoError(t, err)
-
-		issue := unittest.AssertExistsAndLoadBean(t, &issues_model.Issue{Title: "PR with conflict!"})
-		require.NoError(t, issue.LoadPullRequest(db.DefaultContext))
-		conflictingPR := issue.PullRequest
-
-		// Ensure conflictedFiles is populated.
-		assert.Len(t, conflictingPR.ConflictedFiles, 1)
-		// Check if status is correct.
-		assert.Equal(t, issues_model.PullRequestStatusConflict, conflictingPR.Status)
-		// Ensure that mergeable returns false
-		assert.False(t, conflictingPR.Mergeable(db.DefaultContext))
-	})
-}
-
 func TestPullRetargetChildOnBranchDelete(t *testing.T) {
 	onGiteaRun(t, func(t *testing.T, giteaURL *url.URL) {
 		session := loginUser(t, "user1")