Git force branch to be forked from the earlier point

Question

I have have 3 branches: master, b and c:

m1->m2->
        \
         ->b1->b2
                 \
                  ->c1->c2

I want to 'remove' b1 and b2 commits from branch c, but to retain them in branch b:

m1->m2->
        \
         ->b1->b2
          \
           ->c1->c2

How do I can achieve this?