Merge branch 'pf/editor-ignore-sigint'

The behaviour visible to the end users was confusing, when they
attempt to kill a process spawned in the editor that was in turn
launched by Git with SIGINT (or SIGQUIT), as Git would catch that
signal and die.  We ignore these signals now.

* pf/editor-ignore-sigint:
  launch_editor: propagate signals from editor to git
  run-command: do not warn about child death from terminal
  launch_editor: ignore terminal signals while editor has control
  launch_editor: refactor to use start/finish_command
  run-command: drop silent_exec_failure arg from wait_or_whine