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_12/v136.go

@@ -9,7 +9,6 @@ import (
 	"time"
 
 	"gitea.dev/modelmigration/base"
-	repo_model "gitea.dev/models/repo"
 	"gitea.dev/modules/git"
 	"gitea.dev/modules/graceful"
 	"gitea.dev/modules/log"
@@ -83,7 +82,7 @@ func AddCommitDivergenceToPulls(x base.EngineMigration) error {
 				log.Error("Missing base repo with id %d for PR ID %d", pr.BaseRepoID, pr.ID)
 				continue
 			}
-			repoStore := repo_model.CodeRepoByName(baseRepo.OwnerName, baseRepo.Name)
+			repoStore := base.LocalCodeGitRepo(baseRepo.OwnerName, baseRepo.Name)
 			gitRefName := fmt.Sprintf("refs/pull/%d/head", pr.Index)
 			divergence, err := git.GetDivergingCommits(graceful.GetManager().HammerContext(), repoStore, pr.BaseBranch, gitRefName)
 			if err != nil {