staticweb: avoid transferring unchanged files to the webserver

Since we're building websites from scratch on clean environments, even cloning the git repository every time, every file will have a recent modification timestamp. This means rsync will copy every file to the webserver even if it didn't actually change. Also, clients making conditional requests with If-Modified-Since will get the whole file rather than 304 Not Modified.

For example, any tiny typo fix in kde.org will touch every page and image in it, and the hackergotchi images in planet.kde.org will have their modtime touched every hour.

This commit adds -c / --checksum to the rsync command. It makes rsync calculate the hash of the local and remote file, and transfer it only if it actually changed, rather than only comparing size and modtime. That way if a file didn't change in the source git repository, or if the generation process created an file identical to the previous version, it will not be updated during deployment.