C---D =>b1 / / E---F =>b2 | / A--B =======> master | \\ \\ G---H =>b3 \\ I---J =>b4 >
Git has a merging strategy called octupus which can merge multiple branches, as long as there are no complex conflicts. The command to do this is exactly as you suggested git merge b1 b2.
git merge b1 b2