signatures: tell held changes as the screen does

With every message's signatures offered now (1205), Signatures::offer
runs for each, and it asked get_change_meta_by_hash whether the
document holds each signature's change, and each one waiting, up to
4096 of them: Automerge rebuilds the change's metadata from the
document's operations to answer. Ask changes::holds instead (1197),
there and in forks and kept_for; the metadata is still read where it
is used.

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