Refactor git command stdio pipe (#36422)

Most potential deadlock problems should have been fixed, and new code is
unlikely to cause new problems with the new design.

Also raise the minimum Git version required to 2.6.0 (released in 2015)

modules/git/attribute/checker.go

@@ -68,15 +68,14 @@ func CheckAttributes(ctx context.Context, gitRepo *git.Repository, treeish strin
 	}
 	defer cancel()
 
-	stdOut := new(bytes.Buffer)
-	if err := cmd.WithEnv(append(os.Environ(), envs...)).
+	stdout, _, err := cmd.WithEnv(append(os.Environ(), envs...)).
 		WithDir(gitRepo.Path).
-		WithStdout(stdOut).
-		RunWithStderr(ctx); err != nil {
+		RunStdBytes(ctx)
+	if err != nil {
 		return nil, fmt.Errorf("failed to run check-attr: %w", err)
 	}
 
-	fields := bytes.Split(stdOut.Bytes(), []byte{'\000'})
+	fields := bytes.Split(stdout, []byte{'\000'})
 	if len(fields)%3 != 1 {
 		return nil, errors.New("wrong number of fields in return from check-attr")
 	}