Merge branch 'ag/rebase-avoid-unneeded-checkout'

"git rebase -i" (and friends) used to unnecessarily check out the
tip of the branch to be rebased, which has been corrected.

* ag/rebase-avoid-unneeded-checkout:
  rebase -i: stop checking out the tip of the branch to rebase

diff --cc builtin/rebase.c
Simple merge

diff --cc sequencer.c
Simple merge

diff --cc sequencer.h
Simple merge