Initial state:
==============

A - B - C - D - E - F - G - H  master
             \
              \
               I - J - K - L  branchA
                     \
                      \
                       M - N - O branchB



EITHER: After git checkout branchB; git rebase master
(or git rebase master branchB):
=====================================================

A - B - C - D - E - F - G - H  master
             \                \
              \                \
               \               I' - J' - M' - N' - O' branchB
                \
                  I - J - K - L  branchA



OR: After git checkout branchB; git rebase --onto master branchA
(or git rebase --onto master branchA branchB)
================================================================

A - B - C - D - E - F - G - H  master
             \               \
              \               \
               \               M' - N' - O' branchB
                \
                  I - J - K - L  branchA