Merge branch 'diff-table-width-fix' into 'master'

Fixed large comments messing with diff table widths

## What does this MR do?

Fixes the width of the diff table so that the content inside it can't increase or decrease the width of the table cells.

## Screenshots (if relevant)

![Screen_Shot_2016-09-12_at_14.44.44](/uploads/2ca483399d867153e6ae608360ffea5f/Screen_Shot_2016-09-12_at_14.44.44.png)

## What are the relevant issue numbers?

Closes #20694

See merge request !6309

CHANGELOG

@@ -25,6 +25,7 @@ v 8.12.0 (unreleased)
   - Change logo animation to CSS (ClemMakesApps)
   - Instructions for enabling Git packfile bitmaps !6104
   - Use Search::GlobalService.new in the `GET /projects/search/:query` endpoint
+  - Fix long comments in diffs messing with table width
   - Fix pagination on user snippets page
   - Run CI builds with the permissions of users !5735
   - Fix sorting of issues in API

app/assets/stylesheets/pages/diff.scss

@@ -68,6 +68,11 @@
       border-collapse: separate;
       margin: 0;
       padding: 0;
+      table-layout: fixed;
+
+      .diff-line-num {
+        width: 50px;
+      }
 
       .line_holder td {
         line-height: $code_line_height;
@@ -98,10 +103,6 @@
     }
 
     tr.line_holder.parallel {
-      .old_line, .new_line {
-        min-width: 50px;
-      }
-
       td.line_content.parallel {
         width: 46%;
       }