Merge branch 'jc/checkout-merge-base'

* jc/checkout-merge-base:
  rebase -i: teach --onto A...B syntax
  rebase: fix --onto A...B parsing and add tests
  "rebase --onto A...B" replays history on the merge base between A and B
  "checkout A...B" switches to the merge base between A and B

diff --cc builtin-checkout.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc git-rebase--interactive.sh
Simple merge

diff --cc git-rebase.sh
Simple merge

diff --cc sha1_name.c
Simple merge