B-A1-A2-A
  \C1-C2-C

Files touched in both branches A and C since A and C diverged from B. This means all files touched by A1 & A2 compared to the files touched by C1 & C2

# to find the common ancestor commit of branch A & C:
B = git merge-base A C

# to find the common files touched by branches A & C since they diverged from B:

comm -12 $(git diff --name-only B...A | sort) $(git diff --name-only B...C | sort)