fast forward merge overwriting my code

Hello community,

I am new to git, and at the moment I am learning the basics. There are loads of good videos on the internet, but I have one specific question, I haven't found the answer yet:

Let's say I have a .txt file on my master branch. I used

git add .

and

git commit -m "blabla"

so everything is staged and in the history. Now I check out a new branch

git checkout -b testing

and edit the .txt file. I add some new lines at the end, but I also change some of the already existing lines. Then again I add and commit everything. Then I use

git checkout master

and

git merge testing

I would expect git to tell me "hey, wait, you have changed some of the first lines in the .txt file. When you merge, your code on master will be altered". But git just merges everything in. Just imagine this was working code, and changing some of the first lines breaks everything in the following lines. I think I have found out what is the problem: git considers this a fast forward merge (since there were no commits on master between the creation and the merging of the test branch). But this is annoying. I want to be able to choose, what changes I want to keep, when I do the merge (just as in case of a 3way merge, when you can call a graphical merge tool to decide what lines to keep). I know, I could git diff the latest commits hashes of both branches and then fix the file on testing branch accordingly. But those are two separate steps, and I want everything to happen in one convenient step.

Is there any possibility to do so?

Many thanks for any help in advance! Many greetings André Ulrich