Enable the StringReplacement cop.

Also fix one use of `gsub` that would be faster as `delete`.

Use `tr` instead of `gsub` when you are replacing the same number of
characters. Use `delete` instead of `gsub` when you are deleting
characters.

.rubocop.yml

@@ -990,11 +990,12 @@ Performance/RedundantSortBy:
 # string.
 Performance/StartWith:
   Enabled: false
+
 # Use `tr` instead of `gsub` when you are replacing the same number of
 # characters. Use `delete` instead of `gsub` when you are deleting
 # characters.
 Performance/StringReplacement:
-  Enabled: false
+  Enabled: true
 
 # TODO: Enable TimesMap Cop.
 # Checks for `.times.map` calls.

lib/gitlab/diff/parser.rb

@@ -18,7 +18,7 @@ module Gitlab
           @lines.each do |line|
             next if filename?(line)
   
-            full_line = line.gsub(/\n/, '')
+            full_line = line.delete("\n")
   
             if line.match(/^@@ -/)
               type = "match"