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

[Ping Yin <pkufranky@gmail.com>]

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