Support `repo.diff` with just one SnapshotId

[dcherian]

IC can look up the parent for the requested snapshot, so the user need only provide one.

[ianhi]

this is show in git language. which is consistent diff is a comparison, show can act on a single thing

[dcherian]

So should inspect_snapshot show it? Probably not... a diff could be quite large and i might be interested in metadata only.

[ianhi]

I think we've been considering the inspect_* functions to not be cosidered stable API - and this is a very reasonable functino to want.

repo.show(id, metadata_only=True/False)?