enhance: truncate but show long lines in diffs (#39279)

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/gitdiff/gitdiff_parse_test.go

@@ -0,0 +1,48 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package gitdiff
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestTryFixTruncatedString(t *testing.T) {
+	assert.Equal(t, "", tryFixTruncatedString(""))
+
+	t.Run("UTF8", func(t *testing.T) {
+		s := "🌞a🌛"
+		assert.Equal(t, s[:1], tryFixTruncatedString(s[:1]))
+		assert.Equal(t, s[:2], tryFixTruncatedString(s[:2]))
+		assert.Equal(t, s[:3], tryFixTruncatedString(s[:3]))
+		assert.Equal(t, "🌞", tryFixTruncatedString(s[:4]))
+		assert.Equal(t, "🌞a", tryFixTruncatedString(s[:5]))
+		assert.Equal(t, "🌞a", tryFixTruncatedString(s[:6]))
+		assert.Equal(t, "🌞a", tryFixTruncatedString(s[:7]))
+		assert.Equal(t, "🌞a", tryFixTruncatedString(s[:8]))
+		assert.Equal(t, "🌞a🌛", tryFixTruncatedString(s[:9]))
+
+		s = "a🌞🌛"
+		assert.Equal(t, "a", tryFixTruncatedString(s[:1]))
+		assert.Equal(t, "a", tryFixTruncatedString(s[:2]))
+		assert.Equal(t, "a", tryFixTruncatedString(s[:3]))
+		assert.Equal(t, "a", tryFixTruncatedString(s[:4]))
+		assert.Equal(t, "a🌞", tryFixTruncatedString(s[:5]))
+		assert.Equal(t, "a🌞", tryFixTruncatedString(s[:6]))
+	})
+
+	t.Run("Non-UTF8", func(t *testing.T) {
+		s := "\xff\xee\xff\xeeb\xff\xee\xff\xee"
+		assert.Equal(t, "\xff", tryFixTruncatedString(s[:1]))
+		assert.Equal(t, "\xff\xee", tryFixTruncatedString(s[:2]))
+		assert.Equal(t, "\xff\xee\xff", tryFixTruncatedString(s[:3]))
+		assert.Equal(t, "\xff\xee\xff\xee", tryFixTruncatedString(s[:4]))
+		assert.Equal(t, "\xff\xee\xff\xeeb", tryFixTruncatedString(s[:5]))
+		assert.Equal(t, "\xff\xee\xff\xeeb", tryFixTruncatedString(s[:6]))
+		assert.Equal(t, "\xff\xee\xff\xeeb", tryFixTruncatedString(s[:7]))
+		assert.Equal(t, "\xff\xee\xff\xeeb", tryFixTruncatedString(s[:8]))
+		assert.Equal(t, "\xff\xee\xff\xeeb\xff\xee\xff\xee", tryFixTruncatedString(s[:9]))
+	})
+}