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_22/v297.go

@@ -5,12 +5,11 @@ package v1_22
 
 import (
 	"gitea.dev/modelmigration/base"
-	"gitea.dev/models/perm"
 )
 
 func AddRepoUnitEveryoneAccessMode(x base.EngineMigration) error {
 	type RepoUnit struct { //revive:disable-line:exported
-		EveryoneAccessMode perm.AccessMode `xorm:"NOT NULL DEFAULT 0"`
+		EveryoneAccessMode int `xorm:"NOT NULL DEFAULT 0"`
 	}
 	return x.Sync(&RepoUnit{})
 }