zeripath 3aba72c613 Add more checks in migration code (#21011) (#21050)
Backport #21011

When migrating add several more important sanity checks:

* SHAs must be SHAs
* Refs must be valid Refs
* URLs must be reasonable

Signed-off-by: Andrew Thornton <art27@cantab.net>
2022-09-04 08:41:21 -05:00
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