[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/lfs/pointer_scanner.go

@@ -0,0 +1,109 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package lfs
+
+import (
+	"bufio"
+	"context"
+	"io"
+	"strconv"
+	"strings"
+	"sync"
+
+	"code.gitea.io/gitea/modules/git"
+	"code.gitea.io/gitea/modules/git/pipeline"
+)
+
+// SearchPointerBlobs scans the whole repository for LFS pointer files
+func SearchPointerBlobs(ctx context.Context, repo *git.Repository, pointerChan chan<- PointerBlob, errChan chan<- error) {
+	basePath := repo.Path
+
+	catFileCheckReader, catFileCheckWriter := io.Pipe()
+	shasToBatchReader, shasToBatchWriter := io.Pipe()
+	catFileBatchReader, catFileBatchWriter := io.Pipe()
+
+	wg := sync.WaitGroup{}
+	wg.Add(4)
+
+	// Create the go-routines in reverse order.
+
+	// 4. Take the output of cat-file --batch and check if each file in turn
+	// to see if they're pointers to files in the LFS store
+	go createPointerResultsFromCatFileBatch(ctx, catFileBatchReader, &wg, pointerChan)
+
+	// 3. Take the shas of the blobs and batch read them
+	go pipeline.CatFileBatch(ctx, shasToBatchReader, catFileBatchWriter, &wg, basePath)
+
+	// 2. From the provided objects restrict to blobs <=1k
+	go pipeline.BlobsLessThan1024FromCatFileBatchCheck(catFileCheckReader, shasToBatchWriter, &wg)
+
+	// 1. Run batch-check on all objects in the repository
+	if git.CheckGitVersionAtLeast("2.6.0") != nil {
+		revListReader, revListWriter := io.Pipe()
+		shasToCheckReader, shasToCheckWriter := io.Pipe()
+		wg.Add(2)
+		go pipeline.CatFileBatchCheck(ctx, shasToCheckReader, catFileCheckWriter, &wg, basePath)
+		go pipeline.BlobsFromRevListObjects(revListReader, shasToCheckWriter, &wg)
+		go pipeline.RevListAllObjects(ctx, revListWriter, &wg, basePath, errChan)
+	} else {
+		go pipeline.CatFileBatchCheckAllObjects(ctx, catFileCheckWriter, &wg, basePath, errChan)
+	}
+	wg.Wait()
+
+	close(pointerChan)
+	close(errChan)
+}
+
+func createPointerResultsFromCatFileBatch(ctx context.Context, catFileBatchReader *io.PipeReader, wg *sync.WaitGroup, pointerChan chan<- PointerBlob) {
+	defer wg.Done()
+	defer catFileBatchReader.Close()
+
+	bufferedReader := bufio.NewReader(catFileBatchReader)
+	buf := make([]byte, 1025)
+
+loop:
+	for {
+		select {
+		case <-ctx.Done():
+			break loop
+		default:
+		}
+
+		// File descriptor line: sha
+		sha, err := bufferedReader.ReadString(' ')
+		if err != nil {
+			_ = catFileBatchReader.CloseWithError(err)
+			break
+		}
+		sha = strings.TrimSpace(sha)
+		// Throw away the blob
+		if _, err := bufferedReader.ReadString(' '); err != nil {
+			_ = catFileBatchReader.CloseWithError(err)
+			break
+		}
+		sizeStr, err := bufferedReader.ReadString('\n')
+		if err != nil {
+			_ = catFileBatchReader.CloseWithError(err)
+			break
+		}
+		size, err := strconv.Atoi(sizeStr[:len(sizeStr)-1])
+		if err != nil {
+			_ = catFileBatchReader.CloseWithError(err)
+			break
+		}
+		pointerBuf := buf[:size+1]
+		if _, err := io.ReadFull(bufferedReader, pointerBuf); err != nil {
+			_ = catFileBatchReader.CloseWithError(err)
+			break
+		}
+		pointerBuf = pointerBuf[:size]
+		// Now we need to check if the pointerBuf is an LFS pointer
+		pointer, _ := ReadPointerFromBuffer(pointerBuf)
+		if !pointer.IsValid() {
+			continue
+		}
+
+		pointerChan <- PointerBlob{Hash: sha, Pointer: pointer}
+	}
+}