run-command: Allow stderr to be a caller supplied pipe

Like .out, .err may now be set to a file descriptor > 0, which
is a writable pipe/socket/file that the child's stderr will be
redirected into.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.h

@@ -18,7 +18,7 @@ struct child_process {
 	 * - Specify > 0 to set a channel to a particular FD as follows:
 	 *     .in: a readable FD, becomes child's stdin
 	 *     .out: a writable FD, becomes child's stdout/stderr
-	 *     .err > 0 not supported
+	 *     .err: a writable FD, becomes child's stderr
 	 *   The specified FD is closed by start_command(), even in case
 	 *   of errors!
 	 */