Forgot to branch in git, need to move changes from master

git branch -M master my-branch

With a -m or -M option, <oldbranch> will be renamed to <newbranch>. If <oldbranch> had a corresponding reflog, it is renamed to match <newbranch>, and a reflog entry is created to remember the branch renaming. If <newbranch> exists, -M must be used to force the rename to happen.

Source

and then

git fetch origin refs/heads/master:refs/heads/master

or

git branch master my-branch  (or another ref)

git stash
git stash branch <branchname>

If not yet committed anywhere (git status shows a bunch of stuff modified, it's OK if it's "git add"-ed too):

$ git checkout -b newbranch

Despite the name checkout this usage (with -b) does not check anything out. The -b flag says "create a new branch", so git creates the branch-name and makes it correspond to the current HEAD commit. Then it makes HEAD point to the new branch, and stops there.

Your next commit is therefore on newbranch, which has as its parent commit, the commit you were on when you started modifying files. So assuming you were on master, and you had these commits:

A - B - C       <-- HEAD=master

the checkout -b makes this read:

A - B - C       <-- master, HEAD=newbranch

and a later commit adds a new commit D:

A - B - C       <-- master
          \
            D   <-- newbranch