If you have some commits:

    A-B-C-D-E-F-G

Where B is master and origin/master, and you decide you want to make C..G into a topic branch, topicA:

git branch topicA

    A-B-C-D-E-F-G (master)
    A-B-C-D-E-F-G (topicA)

git reset --hard B

    A-B (master)
    A-B-C-D-E-F-G (topicA)

git merge --no-ff topicA

    A-B-----------H (master)
       \         /
        C-D-E-F-G (topicA)