Allow adding new files to an empty repo (#24164)

![image](https://user-images.githubusercontent.com/2114189/232561612-2bfcfd0a-fc04-47ba-965f-5d0bcea46c54.png)

modules/git/command.go

@@ -211,10 +211,18 @@ type RunOpts struct {
 	Env               []string
 	Timeout           time.Duration
 	UseContextTimeout bool
-	Dir               string
-	Stdout, Stderr    io.Writer
-	Stdin             io.Reader
-	PipelineFunc      func(context.Context, context.CancelFunc) error
+
+	// Dir is the working dir for the git command, however:
+	// FIXME: this could be incorrect in many cases, for example:
+	// * /some/path/.git
+	// * /some/path/.git/gitea-data/data/repositories/user/repo.git
+	// If "user/repo.git" is invalid/broken, then running git command in it will use "/some/path/.git", and produce unexpected results
+	// The correct approach is to use `--git-dir" global argument
+	Dir string
+
+	Stdout, Stderr io.Writer
+	Stdin          io.Reader
+	PipelineFunc   func(context.Context, context.CancelFunc) error
 }
 
 func commonBaseEnvs() []string {

modules/git/repo.go

@@ -80,7 +80,7 @@ func InitRepository(ctx context.Context, repoPath string, bare bool) error {
 // IsEmpty Check if repository is empty.
 func (repo *Repository) IsEmpty() (bool, error) {
 	var errbuf, output strings.Builder
-	if err := NewCommand(repo.Ctx, "show-ref", "--head", "^HEAD$").
+	if err := NewCommand(repo.Ctx).AddOptionFormat("--git-dir=%s", repo.Path).AddArguments("show-ref", "--head", "^HEAD$").
 		Run(&RunOpts{
 			Dir:    repo.Path,
 			Stdout: &output,

modules/git/repo_base_nogogit.go

@@ -61,7 +61,7 @@ func OpenRepository(ctx context.Context, repoPath string) (*Repository, error) {
 	}
 
 	repo.batchWriter, repo.batchReader, repo.batchCancel = CatFileBatch(ctx, repoPath)
-	repo.checkWriter, repo.checkReader, repo.checkCancel = CatFileBatchCheck(ctx, repo.Path)
+	repo.checkWriter, repo.checkReader, repo.checkCancel = CatFileBatchCheck(ctx, repoPath)
 
 	return repo, nil
 }