Merge branch 'adam-fix-mr-diff-symlink-file-conversion' into 'master'

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

## What does this MR do?

It fixes a bug described in #21610

## Why was this MR needed?

There was a regression introduced in https://gitlab.com/gitlab-org/gitlab-ce/commit/cd7c2cb6ddd4d9c9f9bdae00c887c0022c121c17

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Fixes #21610

See merge request !6828

CHANGELOG.md

@@ -106,6 +106,7 @@ Please view this file on the master branch, on stable branches it's out of date.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