New command to sync files from an external directory to the Frz repo

[kwiberg]

There should be a command (likely named frz sync) to update the Frz repo with files from a separate directory. The idea is to copy files that are missing or different in the destination, and delete files that are missing in the source—and then frz add all the touched files. The point of having a command for this is to be efficient when determining which files are modified; we should probably do what rsync does (compare file size and modification time).

This would be useful because a lot of applications don't work well with symlinks, so it's useful to have a tree of regular files, which is periodically synced into a Frz repository.