loader mercurial edge case about missing mapping from revision to hgnode-id

Some missing mapping somehow happens which prevent the loader from finishing [2]

I recall seeing that this part is not covered in the loader so we are missing the right test around this to fix it.

ERROR:swh.loader.mercurial.LoaderFromDisk:Loading failure, updating to `failed` status
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/swh/loader/core/loader.py", line 339, in load
    self.store_data()
  File "/usr/lib/python3/dist-packages/swh/loader/mercurial/from_disk.py", line 452, in store_data
    target = self.get_revision_id_from_hg_nodeid(node_id)
  File "/usr/lib/python3/dist-packages/swh/loader/mercurial/from_disk.py", line 526, in get_revision_id_from_hg_nodeid
    assert len(from_storage) == 1, msg % (hg_nodeid.hex(), len(from_storage))
AssertionError: Expected 1 match from storage for hg node '64353138623066626536626164656565336638303062663664343638313639343833643838626338', got 0
{'status': 'failed'}

• [1] swh/meta$1161

• [2] https://sentry.softwareheritage.org/share/issue/342aa3bb1aed46b7bf99efa205f7e081/

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Migrated from T3584 (view on Phabricator)