Move hash, phpcs, and root git commands to ShellOperator

[sirbrillig]

This moves a bunch of git commands to the ShellOperator interface so they can be customized for a different OS. Specifically this moves:

• The code to determine the hash of a git file.
• The code to get the phpcs output of a git file.
• The code to get the git root directory (this was removed actually because I think we can safely always use the git file's full path).
• The code to check if the unmodified file is in git.

This also removes the initial validation of the git file's existence in the repo since that can be handled when we try to fetch the file's full path.

Since this increases the number of times we get the git file's full path, we also cache the full path for the lifetime of the script.

This is part of https://github.com/sirbrillig/phpcs-changed/issues/73