Thanks to these instructions, I do not have any problem gitting and then compiling a single branch.
But: what is the smoothest way to merge a branch (like gtk3) with another (like xtransnew) – without interfering in any way with what is stored on github? In other words: how to perform this merge/compile on my local machine only?
Presently I am doing the merge the hard way, i.e. by copying files from one tree onto another.
Whatever you do will be performed locally on your computer only. You cannot mess our code up 
That’s a relief 
I tried the git merge command, but then github asked me to log in – I did not log in, having fears that I might destroy something… After all, why would he ask me to login if whatever I do is performed locally on my computer only?
Here is what I tried:
Did you fetch xtransnew?
Alternatively, it asked you to log in because the merging is a commit done by you, so it needs to know who did it.
As CarVac wrote.
To merge without making a commit use --no-commit
git merge --help
But no need to merge xtransnew into anything, it has been done already.