show: Add a way to not show a full diff

[craigds]

We'd like to be able to get structured details for a commit without producing or parsing megabytes of JSON.

I'd suggest adding a --diff-format option with the following semantics:

kart show -o json --diff-format=none <sha1>:

No diff at all. display commit metadata only (author/message/parent commit/etc)

kart show -o json --diff-format=full <sha1>

Full diff (same as no flag)

kart show -o json --diff-format=no-data-changes <sha1>

Display a diff, but don't include the feature/tile tree for datasets. Replace with a data_changes: true|false boolean to say if the feature/tile tree changed, so the caller knows whether they might need to request the full diff later.