Add new method to Gitlab::Git::Repository

Add a new convenience method, `#enable_autocrlf`, to the Repository class.  The new method sets the `core.autocrlf` config option to true for a repo.  This can be used by GitLab in satellite repos to handle the `\r\n` line endings that get written when editing via the web interface.  This is part of the fix for gitlabhq/gitlabhq#7950.