replacing a bad commit

[Blu Corater <blu@daga.cl>]

Hello,

Here is the situation. Upstream realeses tarballs once in a while. I
maintain local modifications. Every time upstream releases a tarball, I
fast forward the 'upstream' branch, and merge into 'local' branch. My
tree, currently, looks somewhat like this:

               o---o---o <--topic2
               |
               |  o---o---o <--topic1
               | /
   o---o---C---A---o---o <--local
  /   /   /  
 /   /   /
o---o---o---B <--upstream

Problem is, B should have been merged into 'local', at A, but I just
realized it wasn't (probably due to my own stupidity).

I need to correct A, i.e. merge with B, but keeping the branches already
in flux, and propagating the changes due to the merge to them. In short,
replace A with a properly merged A'.

I tried branching from C and merging with B, then rebasing topic branches,
but then I am not sure how to rebase 'local'to eliminate A.

Thanks in advance for any help.

-- 
Blu.