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>

services/migrations/restore.go

@@ -243,6 +243,7 @@ func (r *RepositoryRestorer) GetPullRequests(page, perPage int) ([]*base.PullReq
 	}
 	for _, pr := range pulls {
 		pr.PatchURL = "file://" + filepath.Join(r.baseDir, pr.PatchURL)
+		CheckAndEnsureSafePR(pr, "", r)
 	}
 	return pulls, true, nil
 }