Discard unread data of git cat-file (#29297)

Fixes #29101
Related #29298

Discard all read data to prevent misinterpreting existing data. Some
discard calls were missing in error cases.

---------

Co-authored-by: yp05327 <576951401@qq.com>
(cherry picked from commit d6811baf88ca6d58b92d4dc12b1f2a292198751f)

modules/git/tree_test.go

@@ -0,0 +1,27 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package git
+
+import (
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSubTree_Issue29101(t *testing.T) {
+	repo, err := openRepositoryWithDefaultContext(filepath.Join(testReposDir, "repo1_bare"))
+	assert.NoError(t, err)
+	defer repo.Close()
+
+	commit, err := repo.GetCommit("ce064814f4a0d337b333e646ece456cd39fab612")
+	assert.NoError(t, err)
+
+	// old code could produce a different error if called multiple times
+	for i := 0; i < 10; i++ {
+		_, err = commit.SubTree("file1.txt")
+		assert.Error(t, err)
+		assert.True(t, IsErrNotExist(err))
+	}
+}