fix(diff): load both blobs of renamed files for diff rendering (#39969) (#39992)

Backport #39969

Co-authored-by: schop-rob <49682405+schop-rob@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/gitdiff/gitdiff_render_test.go

@@ -116,3 +116,40 @@ func TestGetDiffForRender(t *testing.T) {
 		assert.Equal(t, exp, actual, "idx=%d", idx)
 	}
 }
+
+func TestGetDiffForRenderRenamedFile(t *testing.T) {
+	repoDir := filepath.Join(t.TempDir(), "temp-repo")
+	require.NoError(t, git.InitRepositoryLocal(t.Context(), repoDir, false, git.Sha1ObjectFormat.Name()))
+
+	gitRepo, err := git.OpenRepositoryLocal(t.Context(), repoDir)
+	require.NoError(t, err)
+	defer gitRepo.Close()
+
+	content := "id,name\n1,Alice\n2,Bob\n"
+	require.NoError(t, git.ForceFastImport(t.Context(), gitRepo, []git.FastImportCommit{
+		{Ref: "refs/heads/b1", Files: []git.FastImportFile{{Path: "original/sample.csv", Content: content}}},
+		{Ref: "refs/heads/b2", Files: []git.FastImportFile{{Path: "moved/sample.csv", Content: content}}},
+	}))
+
+	beforeCommit, err := gitRepo.GetBranchCommit(t.Context(), "b1")
+	require.NoError(t, err)
+	afterCommit, err := gitRepo.GetBranchCommit(t.Context(), "b2")
+	require.NoError(t, err)
+
+	diff, err := GetDiffForRender(t.Context(), "/any/repo-link", gitRepo, &DiffOptions{
+		BeforeCommitID:    beforeCommit.ID.String(),
+		AfterCommitID:     afterCommit.ID.String(),
+		MaxLines:          setting.Git.MaxGitDiffLines,
+		MaxLineCharacters: setting.Git.MaxGitDiffLineCharacters,
+		MaxFiles:          setting.Git.MaxGitDiffFiles,
+	})
+	require.NoError(t, err)
+	require.Len(t, diff.Files, 1)
+
+	// the CSV diff of a renamed file needs both blobs, otherwise it panics
+	diffFile := diff.Files[0]
+	assert.Equal(t, DiffFileRename, diffFile.Type)
+	assert.True(t, diffFile.IsBlobTypeCsv)
+	assert.NotNil(t, diffFile.LeftBlob)
+	assert.NotNil(t, diffFile.RightBlob)
+}