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.c

@@ -41,7 +41,7 @@ int start_command(struct child_process *cmd)
 		cmd->close_out = 1;
 	}
 
-	need_err = cmd->err < 0;
+	need_err = !cmd->no_stderr && cmd->err < 0;
 	if (need_err) {
 		if (pipe(fderr) < 0) {
 			if (need_in)
@@ -87,7 +87,9 @@ int start_command(struct child_process *cmd)
 			close(cmd->out);
 		}
 
-		if (need_err) {
+		if (cmd->no_stderr)
+			dup_devnull(2);
+		else if (need_err) {
 			dup2(fderr[1], 2);
 			close_pair(fderr);
 		}

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;
 };