[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/blob.go

@@ -5,15 +5,119 @@
 package git
 
 import (
+	"bufio"
 	"bytes"
 	"encoding/base64"
 	"io"
 
+	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/typesniffer"
 	"code.gitea.io/gitea/modules/util"
 )
 
-// This file contains common functions between the gogit and !gogit variants for git Blobs
+// Blob represents a Git object.
+type Blob struct {
+	ID ObjectID
+
+	gotSize bool
+	size    int64
+	name    string
+	repo    *Repository
+}
+
+// DataAsync gets a ReadCloser for the contents of a blob without reading it all.
+// Calling the Close function on the result will discard all unread output.
+func (b *Blob) DataAsync() (io.ReadCloser, error) {
+	wr, rd, cancel := b.repo.CatFileBatch(b.repo.Ctx)
+
+	_, err := wr.Write([]byte(b.ID.String() + "\n"))
+	if err != nil {
+		cancel()
+		return nil, err
+	}
+	_, _, size, err := ReadBatchLine(rd)
+	if err != nil {
+		cancel()
+		return nil, err
+	}
+	b.gotSize = true
+	b.size = size
+
+	if size < 4096 {
+		bs, err := io.ReadAll(io.LimitReader(rd, size))
+		defer cancel()
+		if err != nil {
+			return nil, err
+		}
+		_, err = rd.Discard(1)
+		return io.NopCloser(bytes.NewReader(bs)), err
+	}
+
+	return &blobReader{
+		rd:     rd,
+		n:      size,
+		cancel: cancel,
+	}, nil
+}
+
+// Size returns the uncompressed size of the blob
+func (b *Blob) Size() int64 {
+	if b.gotSize {
+		return b.size
+	}
+
+	wr, rd, cancel := b.repo.CatFileBatchCheck(b.repo.Ctx)
+	defer cancel()
+	_, err := wr.Write([]byte(b.ID.String() + "\n"))
+	if err != nil {
+		log.Debug("error whilst reading size for %s in %s. Error: %v", b.ID.String(), b.repo.Path, err)
+		return 0
+	}
+	_, _, b.size, err = ReadBatchLine(rd)
+	if err != nil {
+		log.Debug("error whilst reading size for %s in %s. Error: %v", b.ID.String(), b.repo.Path, err)
+		return 0
+	}
+
+	b.gotSize = true
+
+	return b.size
+}
+
+type blobReader struct {
+	rd     *bufio.Reader
+	n      int64
+	cancel func()
+}
+
+func (b *blobReader) Read(p []byte) (n int, err error) {
+	if b.n <= 0 {
+		return 0, io.EOF
+	}
+	if int64(len(p)) > b.n {
+		p = p[0:b.n]
+	}
+	n, err = b.rd.Read(p)
+	b.n -= int64(n)
+	return n, err
+}
+
+// Close implements io.Closer
+func (b *blobReader) Close() error {
+	if b.rd == nil {
+		return nil
+	}
+
+	defer b.cancel()
+
+	if err := DiscardFull(b.rd, b.n+1); err != nil {
+		return err
+	}
+
+	b.rd = nil
+
+	return nil
+}
 
 // Name returns name of the tree entry this blob object was created from (or empty string)
 func (b *Blob) Name() string {
@@ -100,3 +204,18 @@ func (b *Blob) GuessContentType() (typesniffer.SniffedType, error) {
 
 	return typesniffer.DetectContentTypeFromReader(r)
 }
+
+// GetBlob finds the blob object in the repository.
+func (repo *Repository) GetBlob(idStr string) (*Blob, error) {
+	id, err := NewIDFromString(idStr)
+	if err != nil {
+		return nil, err
+	}
+	if id.IsZero() {
+		return nil, ErrNotExist{id.String(), ""}
+	}
+	return &Blob{
+		ID:   id,
+		repo: repo,
+	}, nil
+}