Fix syntax coloring on emacs

The rather cryptic:

      "fk_#{Digest::SHA256.hexdigest("#{table}_#{column}_fk").first(10)}"

Was too much for emacs too handle*, since it was coming from the Rails
codebase I took their way of doing the same thing and applied it here.

I think it's easier to read and it also makes emacs render the
migration helpers pretty again.

* not true, emacs can handle anything, leave emacs alone!

lib/gitlab/database/migration_helpers.rb

@@ -225,7 +225,10 @@ module Gitlab
       # here is based on Rails' foreign_key_name() method, which unfortunately
       # is private so we can't rely on it directly.
       def concurrent_foreign_key_name(table, column)
-        "fk_#{Digest::SHA256.hexdigest("#{table}_#{column}_fk").first(10)}"
+        identifier = "#{table_name}_#{column}_fk"
+        hashed_identifier = Digest::SHA256.hexdigest(identifier).first(10)
+
+        "fk_#{hashed_identifier}"
       end
 
       # Long-running migrations may take more than the timeout allowed by