Merge branch 'sp/run-command'

Merge branch 'sp/run-command'

* sp/run-command:
  Use run_command within send-pack
  Use run_command within receive-pack to invoke index-pack
  Use run_command within merge-index
  Use run_command for proxy connections
  Use RUN_GIT_CMD to run push backends
  Correct new compiler warnings in builtin-revert
  Replace fork_with_pipe in bundle with run_command
  Teach run-command to redirect stdout to /dev/null
  Teach run-command about stdout redirection