receive-pack: do not leak output from auto-gc to standard output
The standard output channel of receive-pack is a structured protocol
channel, and subprocesses must never be allowed to leak anything
into it by writing to their standard output.
Use RUN_COMMAND_STDOUT_TO_STDERR option to run_command_v_opt() just
like we do when running hooks to prevent output from "gc" leaking to
the standard output.