SWHIDv2: List issues with SWHIDv1 that should be fixed

(Please edit this task with SWHIDv1 issues)

• The way we format manifests for revisions/releases with negative non-integer timestamps is broken, because "200000 microseconds before timestamp 0" (aka 0 minus 1 seconds plus 800000 microseconds) is represented as "-1.8" in their git-like manifest. Discussed here
• No way to represent missing DAG nodes (discussion at swh/devel/swh-model#1957)
• The same "abstract" object (especially directories) can have many Git representations, therefore multiple Git identifiers, making its SWHIDv1 non-unique (even if one is "more canonical" than others)

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Migrated from T3609 (view on Phabricator)