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>

services/repository/files/cherry_pick.go

@@ -79,21 +79,33 @@ func CherryPick(ctx context.Context, repo *repo_model.Repository, doer *user_mod
 		right, base = base, right
 	}
 
-	description := fmt.Sprintf("CherryPick %s onto %s", right, opts.OldBranch)
-	conflict, _, err := pull.AttemptThreeWayMerge(ctx,
-		t.basePath, t.gitRepo, base, opts.LastCommitID, right, description)
-	if err != nil {
-		return nil, fmt.Errorf("failed to three-way merge %s onto %s: %w", right, opts.OldBranch, err)
-	}
+	var treeHash string
+	if git.SupportGitMergeTree {
+		var conflict bool
+		treeHash, conflict, _, err = pull.MergeTree(ctx, t.gitRepo, base, opts.LastCommitID, right, nil)
+		if err != nil {
+			return nil, fmt.Errorf("failed to three-way merge %s onto %s: %w", right, opts.OldBranch, err)
+		}
 
-	if conflict {
-		return nil, fmt.Errorf("failed to merge due to conflicts")
-	}
+		if conflict {
+			return nil, fmt.Errorf("failed to merge due to conflicts")
+		}
+	} else {
+		description := fmt.Sprintf("CherryPick %s onto %s", right, opts.OldBranch)
+		conflict, _, err := pull.AttemptThreeWayMerge(ctx, t.gitRepo, base, opts.LastCommitID, right, description)
+		if err != nil {
+			return nil, fmt.Errorf("failed to three-way merge %s onto %s: %w", right, opts.OldBranch, err)
+		}
 
-	treeHash, err := t.WriteTree()
-	if err != nil {
-		// likely non-sensical tree due to merge conflicts...
-		return nil, err
+		if conflict {
+			return nil, fmt.Errorf("failed to merge due to conflicts")
+		}
+
+		treeHash, err = t.WriteTree()
+		if err != nil {
+			// likely non-sensical tree due to merge conflicts...
+			return nil, err
+		}
 	}
 
 	// Now commit the tree

services/repository/files/commit.go

@@ -15,7 +15,7 @@ import (
 
 // CountDivergingCommits determines how many commits a branch is ahead or behind the repository's base branch
 func CountDivergingCommits(ctx context.Context, repo *repo_model.Repository, branch string) (*git.DivergeObject, error) {
-	divergence, err := git.GetDivergingCommits(ctx, repo.RepoPath(), repo.DefaultBranch, branch)
+	divergence, err := git.GetDivergingCommits(ctx, repo.RepoPath(), repo.DefaultBranch, branch, nil)
 	if err != nil {
 		return nil, err
 	}