refactor: decouple git.Repository(ctx) from git.Commit & git.Tree (#38464)

1. Storing "ctx" in a long-living object is wrong
2. Make the commit & tree cacheable (for the future performance
optimization)
3. Also fix some bad designs like `// FIXME: bad design, this field can
be nil if the commit is from "last commit cache"`

ref:
* #33893

modules/git/notes.go

@@ -3,6 +3,14 @@
 
 package git
 
+import (
+	"context"
+	"io"
+	"strings"
+
+	"gitea.dev/modules/log"
+)
+
 // NotesRef is the git ref where Gitea will look for git-notes data.
 // The value ("refs/notes/commits") is the default ref used by git-notes.
 const NotesRef = "refs/notes/commits"
@@ -12,3 +20,80 @@ type Note struct {
 	Message []byte
 	Commit  *Commit
 }
+
+// GetNote retrieves the git-notes data for a given commit.
+// FIXME: Add LastCommitCache support
+func GetNote(ctx context.Context, repo *Repository, commitID string, note *Note) error {
+	log.Trace("Searching for git note corresponding to the commit %q in the repository %q", commitID, repo.Path)
+	notes, err := repo.GetCommit(NotesRef)
+	if err != nil {
+		if IsErrNotExist(err) {
+			return err
+		}
+		log.Error("Unable to get commit from ref %q. Error: %v", NotesRef, err)
+		return err
+	}
+
+	path := ""
+
+	tree := notes.Tree()
+	log.Trace("Found tree with ID %q while searching for git note corresponding to the commit %q", tree.ID, commitID)
+
+	var entry *TreeEntry
+	originalCommitID := commitID
+	for len(commitID) > 2 {
+		entry, err = tree.GetTreeEntryByPath(ctx, repo, commitID)
+		if err == nil {
+			path += commitID
+			break
+		}
+		if IsErrNotExist(err) {
+			tree, err = tree.SubTree(ctx, repo, commitID[0:2])
+			path += commitID[0:2] + "/"
+			commitID = commitID[2:]
+		}
+		if err != nil {
+			// Err may have been updated by the SubTree we need to recheck if it's again an ErrNotExist
+			if !IsErrNotExist(err) {
+				log.Error("Unable to find git note corresponding to the commit %q. Error: %v", originalCommitID, err)
+			}
+			return err
+		}
+	}
+
+	blob := entry.Blob(repo)
+	dataRc, err := blob.DataAsync()
+	if err != nil {
+		log.Error("Unable to read blob with ID %q. Error: %v", blob.ID, err)
+		return err
+	}
+	closed := false
+	defer func() {
+		if !closed {
+			_ = dataRc.Close()
+		}
+	}()
+	d, err := io.ReadAll(dataRc)
+	if err != nil {
+		log.Error("Unable to read blob with ID %q. Error: %v", blob.ID, err)
+		return err
+	}
+	_ = dataRc.Close()
+	closed = true
+	note.Message = d
+
+	treePath := ""
+	if idx := strings.LastIndex(path, "/"); idx > -1 {
+		treePath = path[:idx]
+		path = path[idx+1:]
+	}
+
+	lastCommits, err := GetLastCommitForPaths(ctx, repo, notes, treePath, []string{path})
+	if err != nil {
+		log.Error("Unable to get the commit for the path %q. Error: %v", treePath, err)
+		return err
+	}
+	note.Commit = lastCommits[path]
+
+	return nil
+}