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_26/v326_test.go

@@ -11,10 +11,6 @@ import (
 	"gitea.dev/modules/setting"
 	"gitea.dev/modules/test"
 
-	_ "gitea.dev/models/actions"
-	_ "gitea.dev/models/git"
-	_ "gitea.dev/models/repo"
-
 	"github.com/stretchr/testify/require"
 )