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

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



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

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



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

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

(assuming I' is similar enough to I and J' is similar enough
to J that Git can recognize them as equivalent)