In my opinion a very easy and clean way could be:

go back to A

git checkout A

create a new local branch from A

git checkout -b temp

point master's head to this new branch

git symbolic-ref HEAD refs/heads/master

save

git commit

cleanup (delete temp branch)

git branch -D temp

In my opinion a very easy and clean way could be:

go back to A

git checkout -f A

point master's head to the current state

git symbolic-ref HEAD refs/heads/master

save

git commit