We developed in some branches with lot of commits in them and because we where not sure about the probable bugs. After a lot of changes we decide to migrate whole project into master branch without any changes so we select master branch but there is no THEIRS git merge strategy could be found. Using git with CLI doesn't make any difference too. What's the problem?
I found the answer in an untidy way. I copy all of files/directories from my latest commit and go to the master branch and delete old files then replace new files. Finally made a commit and merge latest branch into master in regular way. So I didn't waste any more time to merge conflicts.