Find the "branching off" point between two commits, irrespective of how many times they've been merged into each other.

git-find-cut

#!/bin/bash -e

BRANCH_A="$1"
BRANCH_B="$2"

# Filter out early commits that are not common ancestors
# Also helpfully throws an error if there are *no* common ancestors.
FRONTIER=$(git rev-list --boundary "$BRANCH_A"..."$BRANCH_B" | grep ^- | cut -d- -f2)

# Only happens if branch A == branch B exactly
if [ -z "$FRONTIER" ]; then
  echo "$(git rev-parse "$BRANCH_A")"
  exit 0
fi

# Find topologically-first cut
while read rev; do
  # Check if rev is a cut for both branches
  [ "$(git rev-list --boundary "$BRANCH_A" "$BRANCH_B" "^$rev" | grep ^-)" == "-$rev" ] || continue

  # Found it!
  echo "$rev"
  exit 0
done < <(git rev-list --topo-order $FRONTIER)

# Unreachable
exit 1