How to create a new commit with the content of some commit?

a----b
 \----c

Given the graph above, I want to create a commit b1 on top of c, where b1 and b have the same content. i.e.

a----b
 \----c----b1    ( content(b) == content(b1) )

If there are no untracked files in the working directory, i can do

git checkout b git reset c git add . git commit -m "the copy of b"

Is there any simpler way? And if there are untracked files in the working directory, how to do it?

Ping Yin