Merge branch 'zj-fix-index-rails' into 'master'

Use Ruby methods, not Rails' String#first

See merge request gitlab-org/gitlab-ce!18669

lib/gitlab/git/raw_diff_change.rb

@@ -38,7 +38,9 @@ module Gitlab
       end
 
       def extract_operation
-        case @raw_operation&.first(1)
+        return :unknown unless @raw_operation
+
+        case @raw_operation[0]
         when 'A'
           :added
         when 'C'