rework the html_safe not to use haml's auto escaping

add feature test for inline diff in file header

app/views/projects/diffs/_file_header.html.haml

@@ -17,12 +17,12 @@
     = blob_icon diff_file.b_mode, diff_file.file_path
 
     - if diff_file.renamed_file?
-      - old_path, new_path = mark_inline_diffs(diff_file.old_path, diff_file.new_path)
+      - old_path, new_path = mark_inline_diffs(diff_file.old_path, diff_file.new_path).map(&:html_safe)
       %strong.file-title-name.has-tooltip{ data: { title: diff_file.old_path, container: 'body' } }
-        != old_path
+        = old_path
       →
       %strong.file-title-name.has-tooltip{ data: { title: diff_file.new_path, container: 'body' } }
-        != new_path
+        = new_path
     - else
       %strong.file-title-name.has-tooltip{ data: { title: diff_file.file_path, container: 'body' } }
         = diff_file.file_path

changelogs/unreleased/37576-renamed-files-have-escaped-html-for-the-inline-diff-in-the-header.yml

+---
+title: Fix the diff file header from being html escaped for renamed files.
+merge_request: 14121
+author:
+type: fixed

spec/features/projects/diffs/diff_show_spec.rb

@@ -108,6 +108,19 @@ feature 'Diff file viewer', :js do
     end
   end
 
+  context 'renamed file' do
+    before do
+      visit_commit('6907208d755b60ebeacb2e9dfea74c92c3449a1f')
+    end
+
+    it 'shows the filename with diff highlight' do
+      within('.file-header-content') do
+        expect(page).to have_css('.idiff.left.right.deletion')
+        expect(page).to have_content('files/js/commit.coffee')
+      end
+    end
+  end
+
   context 'binary file that appears to be text in the first 1024 bytes' do
     before do
       # The file we're visiting is smaller than 10 KB and we want it collapsed