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/v1_24/v318.go

@@ -5,14 +5,13 @@ package v1_24
 
 import (
 	"gitea.dev/modelmigration/base"
-	"gitea.dev/models/perm"
 
 	"xorm.io/xorm"
 )
 
 func AddRepoUnitAnonymousAccessMode(x base.EngineMigration) error {
 	type RepoUnit struct { //revive:disable-line:exported
-		AnonymousAccessMode perm.AccessMode `xorm:"NOT NULL DEFAULT 0"`
+		AnonymousAccessMode int `xorm:"NOT NULL DEFAULT 0"`
 	}
 	_, err := x.SyncWithOptions(xorm.SyncOptions{
 		IgnoreConstrains: true,