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_21/v276.go

@@ -7,7 +7,6 @@ import (
 	"context"
 
 	"gitea.dev/modelmigration/base"
-	repo_model "gitea.dev/models/repo"
 	"gitea.dev/modules/git"
 	"gitea.dev/modules/setting"
 )
@@ -159,7 +158,7 @@ func migratePushMirrors(x base.EngineMigration) error {
 
 func getRemoteAddress(ownerName, repoName, remoteName string) (string, error) {
 	ctx := context.Background()
-	repo := repo_model.CodeRepoByName(ownerName, repoName)
+	repo := base.LocalCodeGitRepo(ownerName, repoName)
 	if exist, _ := git.IsRepositoryExist(ctx, repo); !exist {
 		return "", nil
 	}