Merge branch 'en/rebase-merge-on-sequencer'

"git rebase --merge" as been reimplemented by reusing the internal
machinery used for "git rebase -i".

* en/rebase-merge-on-sequencer:
  rebase: implement --merge via the interactive machinery
  rebase: define linearization ordering and enforce it
  git-legacy-rebase: simplify unnecessary triply-nested if
  git-rebase, sequencer: extend --quiet option for the interactive machinery
  am, rebase--merge: do not overlook --skip'ed commits with post-rewrite
  t5407: add a test demonstrating how interactive handles --skip differently
  rebase: fix incompatible options error message
  rebase: make builtin and legacy script error messages the same

git-rebase--common.sh

@@ -10,7 +10,7 @@ write_basic_state () {
 	echo "$head_name" > "$state_dir"/head-name &&
 	echo "$onto" > "$state_dir"/onto &&
 	echo "$orig_head" > "$state_dir"/orig-head &&
-	echo "$GIT_QUIET" > "$state_dir"/quiet &&
+	test t = "$GIT_QUIET" && : > "$state_dir"/quiet
 	test t = "$verbose" && : > "$state_dir"/verbose
 	test -n "$strategy" && echo "$strategy" > "$state_dir"/strategy
 	test -n "$strategy_opts" && echo "$strategy_opts" > \