Is there a tool that can show me what branches a specific branch has been merged into? For example, if "A" has been merged into "B" and "C" but not "D", how do I output "B" and "C"? I'd only need the branch names.
If you mean a graphic tool, gitk
would seem to be a good starting point.
Alternatively, you might prefer the CLI which is very useful for some needs, give it a try :
git log --oneline --decorate --simplify-by-decoration --graph --all
And to adress more specifically the question "How to know which branches are merged into branch A?"
git branch --contains A