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/blob_nogogit.go

@@ -9,7 +9,6 @@ import (
 	"bufio"
 	"bytes"
 	"io"
-	"math"
 
 	"code.gitea.io/gitea/modules/log"
 )
@@ -104,25 +103,6 @@ func (b *blobReader) Read(p []byte) (n int, err error) {
 // Close implements io.Closer
 func (b *blobReader) Close() error {
 	defer b.cancel()
-	if b.n > 0 {
-		for b.n > math.MaxInt32 {
-			n, err := b.rd.Discard(math.MaxInt32)
-			b.n -= int64(n)
-			if err != nil {
-				return err
-			}
-			b.n -= math.MaxInt32
-		}
-		n, err := b.rd.Discard(int(b.n))
-		b.n -= int64(n)
-		if err != nil {
-			return err
-		}
-	}
-	if b.n == 0 {
-		_, err := b.rd.Discard(1)
-		b.n--
-		return err
-	}
-	return nil
+
+	return DiscardFull(b.rd, b.n+1)
 }