Sometimes callers trying to use run_command to execute a child
process will want to setup a pipe or file descriptor to redirect
into the child's stdin.
This idea is completely stolen from builtin-bundle's fork_with_pipe,
written by Johannes Schindelin. All credit (and blame) should lie
with Dscho. ;-)
Signed-off-by: Shawn O. Pearce <spearce@spearce.org> Signed-off-by: Junio C Hamano <junkio@cox.net>