git commit squashing

gistfile1.sh

# Go back to the last commit that we want to form the initial commit (detach HEAD)
git checkout <sha1_for_B>

# reset the branch pointer to the initial commit,
# but leaving the index and working tree intact.
git reset --soft <sha1_for_A>

# amend the initial tree using the tree from 'B'
git commit --amend

# temporarily tag this new initial commit
# (or you could remember the new commit sha1 manually)
git tag tmp

# go back to the original branch (assume master for this example)
git checkout master

# Replay all the commits after B onto the new initial commit
git rebase --onto tmp <sha1_for_B>

# remove the temporary tag
git tag -d tmp