Make the commands less surprising

[mikberg]

When writing docs and relatively thoroughly going through the API now, I found a number of things that were somewhat surprising:

• We are mixing up the idea of a "snapshot name" and the path to the snapshot.
• The snapshot name defaults to git rev-parse HEAD, but this isn't really documented, and feels a bit surprising. This might still be somewhat practical in the collect command, but feels very unintuitive in the push and tag commands.
• The diff outputs just a list of targets by default. I think most will use the JSON output format.
• The collect command can optionally also push the snapshot to remote storage, and I think it's debatable what value this brings. I think the clarity of doing this in two commands would be preferable. As far as I know, no-one is using this feature.