What does git H do?

It's not git -H, it's an option to sudo. Read this as:

sudo         // This is what we're running
-u git       // Run the command specified later as user "git"
-H bundle    // Set the HOME environment variable to that of user "bundle"
exec rake gitlab:backup:create // The command to execute

If sudo is complaining that it doesn't know what -H means, that may mean you have an old version of sudo that doesn't support it, or that your flavour of sudo uses a different command line option for the same behaviour. I'd suggest checking for updates, and then using man sudo to check for the exact version you're running locally and which options it accepts.