By default, rebase will take all commits from the branch that is to be
rebased which are missing in upstream. The new --first-parent option
allows to just follow the first parent and thus completely ignore
merges.

Additionally, when used together with --preserve-merges (which is the
more useful use-case) it will no longer rebase the commits from the
merged-in branches, but instead redo the merge with the original
parents.

That means that:
     ---H------I topicB
    /    \      \
    | D---E---F--G topicA
    |/
    A---B---C master

does no longer become:
                -H'--------I'
               /  \         \
              D'---E'---F'---G' topicA
             /
    A---B---C master
     \
      H---I topicB

but instead:
    A---B---C master
     \       \
      \       D'---E'---F'---G' topicA
       \          /         /
        ---------H---------I topicB

Signed-off-by: Bj