refactor: use WithRepo instead of WithDir for most git operations, clean up model migrations (#38555)

by the way, remove some unnecessary "models" imports from model
migration package, fix migration test model init bug
(modelmigration/migrationtest/tests.go)

modelmigration/migrationtest/tests.go

@@ -58,7 +58,11 @@ func PrepareTestEnv(t *testing.T, skip int, syncModels ...any) (base.EngineMigra
 		}
 	}
 
+	db.ResetModels()
 	if len(syncModels) > 0 {
+		for _, syncModel := range syncModels {
+			db.RegisterModel(syncModel)
+		}
 		if err := x.Sync(syncModels...); err != nil {
 			t.Errorf("error during sync: %v", err)
 			return x, deferFn