579
579
raise errors.InvalidRevisionSpec(self.user_spec, context_branch,
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580
'cannot go before the null: revision')
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581
context_repo = context_branch.repository
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584
parent_map = context_repo.get_parent_map([base_revision_id])
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587
if base_revision_id not in parent_map:
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588
# Ghost, or unknown revision id
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589
raise errors.InvalidRevisionSpec(self.user_spec, context_branch,