Merge branch '20510-fix-dont-pass-a-local-variable-called-to-a-partial' into 'master'

Fix Mystery Guest

## What does this MR do?
Makes the name of the local variable more expressive rather than using a character, which is the problem of the magic variable/mystery guest.

## Are there points in the code the reviewer needs to double check?
Only the partial that receives the new name of the local variable is being called.

## Why was this MR needed?
Ensure that the pass of local variable don't use a magic name.

## What are the relevant issue numbers?
Closes #20510.

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] All builds are passing
- [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 you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5657

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.11.0 (unreleased)
+  - Fix don't pass a local variable called `i` to a partial. !20510 (herminiotorres)
   - Fix the title of the toggle dropdown button. !5515 (herminiotorres)
   - Improve diff performance by eliminating redundant checks for text blobs
   - Convert switch icon into icon font (ClemMakesApps)

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

@@ -29,5 +29,5 @@
     - next unless blob
     - blob.load_all_data!(diffs.project.repository) unless blob.only_display_raw?
 
-    = render 'projects/diffs/file', i: index, project: diffs.project,
+    = render 'projects/diffs/file', index: index, project: diffs.project,
       diff_file: diff_file, diff_commit: diff_commit, blob: blob

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

-.diff-file.file-holder{id: "diff-#{i}", data: diff_file_html_data(project, diff_file)}
+.diff-file.file-holder{id: "diff-#{index}", data: diff_file_html_data(project, diff_file)}
   .file-title{id: "file-path-#{hexdigest(diff_file.file_path)}"}
-    = render "projects/diffs/file_header", diff_file: diff_file, blob: blob, diff_commit: diff_commit, project: project, url: "#diff-#{i}"
+    = render "projects/diffs/file_header", diff_file: diff_file, blob: blob, diff_commit: diff_commit, project: project, url: "#diff-#{index}"
 
     - unless diff_file.submodule?
       .file-actions.hidden-xs