Following a clever trick shown to me by @L0veSunshine use the SHA of an empty Tree

See go-gitea#11674 (comment)

Co-Authored-By: L0veSunshine <xuan199651@gmail.com>
Signed-off-by: Andrew Thornton <art27@cantab.net>

Author: zeripath

modules/git/repo_compare.go

@@ -122,11 +122,11 @@ func (repo *Repository) GetDiffNumChangedFiles(base, head string) (int, error) {
 
 // GetDiffShortStat counts number of changed files, number of additions and deletions
 func (repo *Repository) GetDiffShortStat(base, head string) (numFiles, totalAdditions, totalDeletions int, err error) {
-	return GetDiffShortStat(nil, repo.Path, base+"..."+head)
+	return GetDiffShortStat(repo.Path, base+"..."+head)
 }
 
 // GetDiffShortStat counts number of changed files, number of additions and deletions
-func GetDiffShortStat(env []string, repoPath string, args ...string) (numFiles, totalAdditions, totalDeletions int, err error) {
+func GetDiffShortStat(repoPath string, args ...string) (numFiles, totalAdditions, totalDeletions int, err error) {
 	// Now if we call:
 	// $ git diff --shortstat 1ebb35b98889ff77299f24d82da426b434b0cca0...788b8b1440462d477f45b0088875
 	// we get:
@@ -136,7 +136,7 @@ func GetDiffShortStat(env []string, repoPath string, args ...string) (numFiles,
 		"--shortstat",
 	}, args...)
 
-	stdout, err := NewCommand(args...).RunInDirWithEnv(repoPath, env)
+	stdout, err := NewCommand(args...).RunInDir(repoPath)
 	if err != nil {
 		return 0, 0, 0, err
 	}

modules/git/sha1.go

@@ -17,6 +17,9 @@ import (
 // EmptySHA defines empty git SHA
 const EmptySHA = "0000000000000000000000000000000000000000"
 
+// EmptyTreeSHA is the SHA of an empty tree
+const EmptyTreeSHA = "4b825dc642cb6eb9a060e54bf8d69288fbee4904"
+
 // SHAPattern can be used to determine if a string is an valid sha
 var SHAPattern = regexp.MustCompile(`^[0-9a-f]{4,40}$`)
 

modules/repofiles/temp_repo.go

@@ -299,7 +299,7 @@ func (t *TemporaryUploadRepository) DiffIndex() (*gitdiff.Diff, error) {
 			t.repo.FullName(), err, stderr)
 	}
 
-	diff.NumFiles, diff.TotalAddition, diff.TotalDeletion, err = git.GetDiffShortStat(nil, t.basePath, "--cached", "HEAD")
+	diff.NumFiles, diff.TotalAddition, diff.TotalDeletion, err = git.GetDiffShortStat(t.basePath, "--cached", "HEAD")
 	if err != nil {
 		return nil, err
 	}

services/gitdiff/gitdiff.go

@@ -17,7 +17,6 @@ import (
 	"net/url"
 	"os"
 	"os/exec"
-	"path/filepath"
 	"sort"
 	"strconv"
 	"strings"
@@ -713,20 +712,10 @@ func GetDiffRangeWithWhitespaceBehavior(repoPath, beforeCommitID, afterCommitID
 	}
 
 	shortstatArgs := []string{beforeCommitID + "..." + afterCommitID}
-	var env []string
 	if len(beforeCommitID) == 0 || beforeCommitID == git.EmptySHA {
-		shortstatArgs = []string{"--cached", "-R", afterCommitID}
-		tmpDir, err := ioutil.TempDir("", "gitdiff")
-		if err != nil {
-			return nil, err
-		}
-		defer func() {
-			_ = os.RemoveAll(tmpDir)
-		}()
-		env = append(os.Environ(), "GIT_INDEX_FILE="+filepath.Join(tmpDir, "non-existent-index"))
-
+		shortstatArgs = []string{git.EmptyTreeSHA, afterCommitID}
 	}
-	diff.NumFiles, diff.TotalAddition, diff.TotalDeletion, err = git.GetDiffShortStat(env, repoPath, shortstatArgs...)
+	diff.NumFiles, diff.TotalAddition, diff.TotalDeletion, err = git.GetDiffShortStat(repoPath, shortstatArgs...)
 	if err != nil {
 		return nil, err
 	}