Use Ruby methods, not Rails' String#first

Gitaly-Ruby is not a Rails application, which made the method not exist
after the vendor. This patch uses direct indexing, which has the same
properties that `#first` has.

Found while working on: https://gitlab.com/gitlab-org/gitaly/merge_requests/699

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'