* 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