Disable error message from `:Cancel` on git commit

[atusy]

Currently, :Cancel followed by :Gin commit shows following error message.

As long as users intentionally exec :Cancel, they do not need the above message, IMO.

At the same time, I understand this is compatible with doing :cq when vim opens with git commit.

> GIT_EDITOR="vim -u NORC -c 'cq'" git commit --allow-empty
hint: Waiting for your editor to close the file... error: There was a problem with the editor 'vim -u NORC -c 'cq''.
Please supply the message using either -m or -F option.