run-command: Support sending stderr to /dev/null

Some callers may wish to redirect stderr to /dev/null in some
contexts, such as if they are executing a command only to get
the exit status and don't want users to see whatever output it
may produce as a side-effect of computing that exit status.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

run-command.h

@@ -23,6 +23,7 @@ struct child_process {
 	unsigned close_out:1;
 	unsigned no_stdin:1;
 	unsigned no_stdout:1;
+	unsigned no_stderr:1;
 	unsigned git_cmd:1; /* if this is to be git sub-command */
 	unsigned stdout_to_stderr:1;
 };