run-command: Redirect stderr to a pipe before redirecting stdout to stderr

run-command: Redirect stderr to a pipe before redirecting stdout to stderr

With this patch, in the 'start_command' function after forking
we now take care of stderr in the child process before stdout.

This way if 'start_command' is called with a 'child_process'
argument like this:

	.err = -1;
	.stdout_to_stderr = 1;

then stderr will be redirected to a pipe before stdout is
redirected to stderr. So we can now get the process' stdout
from the pipe (as well as its stderr).

Earlier such a call would have redirected stdout to stderr
before stderr was itself redirected, and therefore stdout
would not have followed stderr, which would not have been
very useful anyway.

Update documentation in 'api-run-command.txt' accordingly.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Acked-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>