refactor: simplify ParseCatFileTreeLine and catBatchParseTreeEntries (#37210)

Simplify ParseCatFileTreeLine: it is faster without the preset buffers,
and easier to read and maintain.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: wxiaoguang <2114189+wxiaoguang@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/git/parse_treeentry_test.go

@@ -4,6 +4,9 @@
 package git
 
 import (
+	"bufio"
+	"io"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -100,3 +103,31 @@ func TestParseTreeEntriesInvalid(t *testing.T) {
 	assert.Error(t, err)
 	assert.Empty(t, entries)
 }
+
+func TestParseCatFileTreeLine(t *testing.T) {
+	input := "100644 looooooooooooooooooooooooong-file-name.txt\x0012345678901234567890"
+	input += "40755 some-directory\x00abcdefg123abcdefg123"
+
+	var readCount int
+
+	buf := bufio.NewReaderSize(strings.NewReader(input), 20) // NewReaderSize has a limit: min buffer size = 16
+	mode, name, objID, n, err := ParseCatFileTreeLine(Sha1ObjectFormat, buf)
+	readCount += n
+	assert.NoError(t, err)
+	assert.Equal(t, EntryModeBlob, mode)
+	assert.Equal(t, "looooooooooooooooooooooooong-file-name.txt", name)
+	assert.Equal(t, "12345678901234567890", string(objID.RawValue()))
+
+	mode, name, objID, n, err = ParseCatFileTreeLine(Sha1ObjectFormat, buf)
+	readCount += n
+	assert.NoError(t, err)
+	assert.Equal(t, EntryModeTree, mode)
+	assert.Equal(t, "some-directory", name)
+	assert.Equal(t, "abcdefg123abcdefg123", string(objID.RawValue()))
+
+	assert.Equal(t, len(input), readCount)
+
+	_, _, _, n, err = ParseCatFileTreeLine(Sha1ObjectFormat, buf)
+	assert.ErrorIs(t, err, io.EOF)
+	assert.Zero(t, n)
+}