Merge branch 'dl/rebase-i-keep-base'

Merge branch 'dl/rebase-i-keep-base'

"git rebase --keep-base <upstream>" tries to find the original base
of the topic being rebased and rebase on top of that same base,
which is useful when running the "git rebase -i" (and its limited
variant "git rebase -x").

The command also has learned to fast-forward in more cases where it
can instead of replaying to recreate identical commits.

* dl/rebase-i-keep-base:
  rebase: teach rebase --keep-base
  rebase tests: test linear branch topology
  rebase: fast-forward --fork-point in more cases
  rebase: fast-forward --onto in more cases
  rebase: refactor can_fast_forward into goto tower
  t3432: test for --no-ff's interaction with fast-forward
  t3432: distinguish "noop-same" v.s. "work-same" in "same head" tests
  t3432: test rebase fast-forward behavior
  t3431: add rebase --fork-point tests