git rebase --interactive HEAD~2

#  we are going to squash c into b 
pick b76d157 b
pick a931ac7 c

# squash c into b
pick b76d157 b
s a931ac7 c

# after that just edit the commit message

# This is a combination of 2 commits.
# The first commit's message is:

b

# This is the 2nd commit message:

c