[CHORE] Drop go-git support

See https://codeberg.org/forgejo/discussions/issues/164 for the
rationale and discussion of this change.

Everything related to the `go-git` dependency is dropped (Only a single
instance is left in a test file to test for an XSS, it requires crafting
an commit that Git itself refuses to craft). `_gogit` files have
been removed entirely, `go:build: !gogit` is removed, `XXX_nogogit.go` files
either have been renamed or had their code being merged into the
`XXX.go` file.

modules/git/repo_tree.go

@@ -6,6 +6,7 @@ package git
 
 import (
 	"bytes"
+	"io"
 	"os"
 	"strings"
 	"time"
@@ -65,3 +66,88 @@ func (repo *Repository) CommitTree(author, committer *Signature, tree *Tree, opt
 	}
 	return NewIDFromString(strings.TrimSpace(stdout.String()))
 }
+
+func (repo *Repository) getTree(id ObjectID) (*Tree, error) {
+	wr, rd, cancel := repo.CatFileBatch(repo.Ctx)
+	defer cancel()
+
+	_, _ = wr.Write([]byte(id.String() + "\n"))
+
+	// ignore the SHA
+	_, typ, size, err := ReadBatchLine(rd)
+	if err != nil {
+		return nil, err
+	}
+
+	switch typ {
+	case "tag":
+		resolvedID := id
+		data, err := io.ReadAll(io.LimitReader(rd, size))
+		if err != nil {
+			return nil, err
+		}
+		tag, err := parseTagData(id.Type(), data)
+		if err != nil {
+			return nil, err
+		}
+		commit, err := tag.Commit(repo)
+		if err != nil {
+			return nil, err
+		}
+		commit.Tree.ResolvedID = resolvedID
+		return &commit.Tree, nil
+	case "commit":
+		commit, err := CommitFromReader(repo, id, io.LimitReader(rd, size))
+		if err != nil {
+			return nil, err
+		}
+		if _, err := rd.Discard(1); err != nil {
+			return nil, err
+		}
+		commit.Tree.ResolvedID = commit.ID
+		return &commit.Tree, nil
+	case "tree":
+		tree := NewTree(repo, id)
+		tree.ResolvedID = id
+		objectFormat, err := repo.GetObjectFormat()
+		if err != nil {
+			return nil, err
+		}
+		tree.entries, err = catBatchParseTreeEntries(objectFormat, tree, rd, size)
+		if err != nil {
+			return nil, err
+		}
+		tree.entriesParsed = true
+		return tree, nil
+	default:
+		if err := DiscardFull(rd, size+1); err != nil {
+			return nil, err
+		}
+		return nil, ErrNotExist{
+			ID: id.String(),
+		}
+	}
+}
+
+// GetTree find the tree object in the repository.
+func (repo *Repository) GetTree(idStr string) (*Tree, error) {
+	objectFormat, err := repo.GetObjectFormat()
+	if err != nil {
+		return nil, err
+	}
+	if len(idStr) != objectFormat.FullLength() {
+		res, err := repo.GetRefCommitID(idStr)
+		if err != nil {
+			return nil, err
+		}
+		if len(res) > 0 {
+			idStr = res
+		}
+	}
+	id, err := NewIDFromString(idStr)
+	if err != nil {
+		return nil, err
+	}
+
+	return repo.getTree(id)
+}