$ git checkout branchX
$ git rebase --onto master master@{1}

$ limit=$(git rev-list --topo-order --cherry master...branchX |
  sed -n -e 's/=//p' | head -1)
$ echo $limit    # if this is empty, there's no equivalent commit and you are SOL
$ git checkout branchX       # same as before
$ git rebase --onto master $limit

A--B--C--D--E   <-- master

              F--G   <-- branchX

D'-E'  <-- master
    /
A--B--C--D--E   [master was this before the copies]

              F--G   <-- branchX

A..B = (reachable(A) | reachable(B)) - (reachable(A) & reachable(B))

D'-E'  <-- master
    /
A--B--C--D--E--F--G   <-- branchX

$ git rev-list --cherry-mark master...two
=dcbcb2774954437ef0906c6770c7deb924d9286e
+0af7c6a3cf5e49928de132c341c848be80ab84c7
=643b37ef242fdc35dfdd4551b42393af3eb91a85

$ git rev-list --left-right master...two
>dcbcb2774954437ef0906c6770c7deb924d9286e
<0af7c6a3cf5e49928de132c341c848be80ab84c7
<643b37ef242fdc35dfdd4551b42393af3eb91a85

$ git rev-list --left-only --cherry-mark master...two
+0af7c6a3cf5e49928de132c341c848be80ab84c7
=643b37ef242fdc35dfdd4551b42393af3eb91a85

$ git rev-list --right-only --cherry-mark master...two
=dcbcb2774954437ef0906c6770c7deb924d9286e

$ git rev-list --cherry master...branchX

$ git rev-list --cherry master...branchX | sed -n -e 's/=//p'

$ limit=$(git rev-list --topo-order --cherry master...branchX |
  sed -n -e 's/=//p' | head -1)

$ git checkout branchX    # if needed
$ git rebase --onto master $limit