Safely mark deleted Git entries as INVALID/withdrawn (and correctly handle undeletion)

[andrewpollock]

There are expected situations where records in Git will be deleted, e.g. bitnami/vulndb#336 (comment)

#829 completely removed the behaviour of marking deleted Git entries as invalid, as a reaction to an upstream defect that caused all records for that source to temporarily disappear.

a) Revert #829, and add guardrails to refuse to process a source with more than (say) 10% of the records disappearing. This would have prevented what happened that caused #829 to be created. Refer to the guardrails as added as part of #2030 for inspiration

b) Ensure the importer can correctly transition an existing deleted and withdrawn Bug back to a non-withdrawn state if it gets undeleted.