Merge branch 'mm/rebase-i-exec'

Merge branch 'mm/rebase-i-exec'

* mm/rebase-i-exec:
  git-rebase--interactive.sh: use printf instead of echo to print commit message
  git-rebase--interactive.sh: rework skip_unnecessary_picks
  test-lib: user-friendly alternatives to test [-d|-f|-e]
  rebase -i: add exec command to launch a shell command

Conflicts:
	git-rebase--interactive.sh
	t/t3404-rebase-interactive.sh