Merge branch 'en/rebase-backend'

Merge branch 'en/rebase-backend'

"git rebase" has learned to use the merge backend (i.e. the
machinery that drives "rebase -i") by default, while allowing
"--apply" option to use the "apply" backend (e.g. the moral
equivalent of "format-patch piped to am").  The rebase.backend
configuration variable can be set to customize.

* en/rebase-backend:
  rebase: rename the two primary rebase backends
  rebase: change the default backend from "am" to "merge"
  rebase: make the backend configurable via config setting
  rebase tests: repeat some tests using the merge backend instead of am
  rebase tests: mark tests specific to the am-backend with --am
  rebase: drop '-i' from the reflog for interactive-based rebases
  git-prompt: change the prompt for interactive-based rebases
  rebase: add an --am option
  rebase: move incompatibility checks between backend options a bit earlier
  git-rebase.txt: add more details about behavioral differences of backends
  rebase: allow more types of rebases to fast-forward
  t3432: make these tests work with either am or merge backends
  rebase: fix handling of restrict_revision
  rebase: make sure to pass along the quiet flag to the sequencer
  rebase, sequencer: remove the broken GIT_QUIET handling
  t3406: simplify an already simple test
  rebase (interactive-backend): fix handling of commits that become empty
  rebase (interactive-backend): make --keep-empty the default
  t3404: directly test the behavior of interest
  git-rebase.txt: update description of --allow-empty-message