Fix the diff in the merge request view when converting a symlink to a regular file.

In this specific case using file_path as a cache key is not enough,
because there are two entries with the same path.
Closes #21610.

CHANGELOG.md

@@ -103,6 +103,7 @@ Please view this file on the master branch, on stable branches it's out of date.
   - Reduce queries needed to find users using their SSH keys when pushing commits
   - Prevent rendering the link to all when the author has no access (Katarzyna Kobierska Ula Budziszewska)
   - Fix broken repository 500 errors in project list
+  - Fix the diff in the merge request view when converting a symlink to a regular file
   - Fix Pipeline list commit column width should be adjusted
   - Close todos when accepting merge requests via the API !6486 (tonygambone)
   - Ability to batch assign issues relating to a merge request to the author. !5725 (jamedjo)

lib/gitlab/diff/file.rb

@@ -125,6 +125,10 @@ module Gitlab
 
         repository.blob_at(commit.id, file_path)
       end
+
+      def cache_key
+        "#{file_path}-#{new_file}-#{deleted_file}-#{renamed_file}"
+      end
     end
   end
 end

lib/gitlab/diff/file_collection/merge_request_diff.rb

@@ -35,16 +35,16 @@ module Gitlab
         # for the highlighted ones, so we just skip their execution.
         # If the highlighted diff files lines are not cached we calculate and cache them.
         #
-        # The content of the cache is a Hash where the key correspond to the file_path and the values are Arrays of
+        # The content of the cache is a Hash where the key identifies the file and the values are Arrays of
         # hashes that represent serialized diff lines.
         #
         def cache_highlight!(diff_file)
-          file_path = diff_file.file_path
+          item_key = diff_file.cache_key
 
-          if highlight_cache[file_path]
-            highlight_diff_file_from_cache!(diff_file, highlight_cache[file_path])
+          if highlight_cache[item_key]
+            highlight_diff_file_from_cache!(diff_file, highlight_cache[item_key])
           else
-            highlight_cache[file_path] = diff_file.highlighted_diff_lines.map(&:to_hash)
+            highlight_cache[item_key] = diff_file.highlighted_diff_lines.map(&:to_hash)
           end
         end