Merge branch 'jk/run-command-use-shell'

* jk/run-command-use-shell:
  t4030, t4031: work around bogus MSYS bash path conversion
  diff: run external diff helper with shell
  textconv: use shell to run helper
  editor: use run_command's shell feature
  run-command: optimize out useless shell calls
  run-command: convert simple callsites to use_shell
  t0021: use $SHELL_PATH for the filter script
  run-command: add "use shell" option

diff --cc diff.c
Simple merge