run-command: add new check_command helper

And persistent_waitpid() to recover the information from the last run.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
transport-helper: check if remote helper is alive

Otherwise transport-helper will continue checking for refs and other
things what will confuse the user more.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'fc/transport-helper-waitpid' into pu

* fc/transport-helper-waitpid:
  transport-helper: check if remote helper is alive
  run-command: add new check_command helper
Merge branch 'jk/rm-removed-paths' into pu

* jk/rm-removed-paths:
  t3600: test rm of path with changed leading symlinks
  t3600: test behavior of reverse-d/f conflict
  rm: do not complain about d/f conflicts during deletion