Use gitrepo's clone and push when possible (#36093)

1 Move `IsRepositoryModelOrDirExist` and `CheckCreateRepository` to
service layer
2 Use `gitrepo.Pushxxx` instead of `git.Push` when possible
3 use `gitrepo.Clonexxx` instead of `gitrepo.Clone` when possible

services/repository/merge_upstream.go

@@ -11,6 +11,7 @@ import (
 	repo_model "code.gitea.io/gitea/models/repo"
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/git"
+	"code.gitea.io/gitea/modules/gitrepo"
 	repo_module "code.gitea.io/gitea/modules/repository"
 	"code.gitea.io/gitea/modules/reqctx"
 	"code.gitea.io/gitea/modules/util"
@@ -33,8 +34,7 @@ func MergeUpstream(ctx reqctx.RequestContext, doer *user_model.User, repo *repo_
 		return "up-to-date", nil
 	}
 
-	err = git.Push(ctx, repo.BaseRepo.RepoPath(), git.PushOptions{
-		Remote: repo.RepoPath(),
+	err = gitrepo.Push(ctx, repo.BaseRepo, repo, git.PushOptions{
 		Branch: fmt.Sprintf("%s:%s", divergingInfo.BaseBranchName, branch),
 		Env:    repo_module.PushingEnvironment(doer, repo),
 	})