fdo#70625 Add --cat parameter to make git diffs pretty

Change-Id: I309ca3e718a11ca97991686c229c79ab20e8c3d9
Reviewed-on: https://gerrit.libreoffice.org/10623
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>