Fix computed data not assigned to cache

In Commit.php and Tag.php the computed value is stored in the class variable $data but in a (non existing) local $data variable. Causing recompute when the function is called again.

Fix assigning two data types to the same variable

In CommitParser.php, LogParser.php and TagParser.php a function is called to get the date as a string and assigned to the class variable, only to be parsed as a \DateTime a line later and assigned to the same variable.

Solution used here to to create a temporary variable and parse that as a sate to avoid the class variable being two types.

Fix calling of removed functions that would just return null

In ReferenceBag.php the initialize has an try around run and the catch expect functions to be available on RuntimeException that where removed and just return null.

This seems to be behavior that is now expected. So if it throws an error a null is used instead of using functions that do not exists.

Please double check this last one. It resembles the what is currently happening, but do not know you want to change this.