fixes #2220

issue #1797

Improved `QueryBuilder::buildLimit()` to support big numbers

added quick and dirty code highlighing and got much faster

realpath(false) = current working directory.
This can cause problems with getAlias() which returns false.

see yiisoft/yii#3113

- Gii diff markup is now copy paste friendly
- Removed StringHelper::diff()
- Moved phpspec/php-diff dependency from yiisoft/yii2 to yiisoft/yii2-gii