"git rebase --rebase-merges" learned to drive different merge
strategies and pass strategy specific options to them.

* js/rebase-r-strategy:
  t3427: accelerate this test by using fast-export and fast-import
  rebase -r: do not (re-)generate root commits with `--root` *and* `--onto`
  t3418: test `rebase -r` with merge strategies
  t/lib-rebase: prepare for testing `git rebase --rebase-merges`
  rebase -r: support merge strategies other than `recursive`
  t3427: fix another incorrect assumption
  t3427: accommodate for the `rebase --merge` backend having been replaced
  t3427: fix erroneous assumption
  t3427: condense the unnecessarily repetitive test cases into three
  t3427: move the `filter-branch` invocation into the `setup` case
  t3427: simplify the `setup` test case significantly
  t3427: add a clarifying comment
  rebase: fold git-rebase--common into the -p backend
  sequencer: the `am` and `rebase--interactive` scripts are gone
  .gitignore: there is no longer a built-in `git-rebase--interactive`
  t3400: stop referring to the scripted rebase
  Drop unused git-rebase--am.sh