I started working on a new feature on master, by creating a commit A:
o -- A  (master)
before realizing that I should probably create a new branch. However, I didn't "move" the commit A to the new branch, so now my repo looks like this:
o -- A (master)
      \ 
       B - C - D - E (feature)
Luckily, I did not push to remote, nor are there any commits after o on master. How to move A from master to feature branch, and make my history look like this:
o (master)
 \
  A - B - C - D - E (feature)