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>

modules/migration/repo.go

@@ -12,7 +12,7 @@ type Repository struct {
 	IsPrivate     bool `yaml:"is_private"`
 	IsMirror      bool `yaml:"is_mirror"`
 	Description   string
-	CloneURL      string `yaml:"clone_url"`
+	CloneURL      string `yaml:"clone_url"` // SECURITY: This must be checked to ensure that is safe to be used
 	OriginalURL   string `yaml:"original_url"`
 	DefaultBranch string
 }