trailer: use capture_command

When we read from a trailer.*.command sub-program, the
current code uses run_command followed by a pipe read, which
can result in deadlock (though in practice you would have to
have a large trailer for this to be a problem). The current
code also leaks the file descriptor for the pipe to the
sub-command.

Instead, let's use capture_command, which makes this simpler
(and we can get rid of our custom helper).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

trailer.c

@@ -214,16 +214,6 @@ static struct trailer_item *remove_first(struct trailer_item **first)
 	return item;
 }
 
-static int read_from_command(struct child_process *cp, struct strbuf *buf)
-{
-	if (run_command(cp))
-		return error("running trailer command '%s' failed", cp->argv[0]);
-	if (strbuf_read(buf, cp->out, 1024) < 1)
-		return error("reading from trailer command '%s' failed", cp->argv[0]);
-	strbuf_trim(buf);
-	return 0;
-}
-
 static const char *apply_command(const char *command, const char *arg)
 {
 	struct strbuf cmd = STRBUF_INIT;
@@ -240,14 +230,16 @@ static const char *apply_command(const char *command, const char *arg)
 	cp.argv = argv;
 	cp.env = local_repo_env;
 	cp.no_stdin = 1;
-	cp.out = -1;
 	cp.use_shell = 1;
 
-	if (read_from_command(&cp, &buf)) {
+	if (capture_command(&cp, &buf, 1024)) {
+		error("running trailer command '%s' failed", cmd.buf);
 		strbuf_release(&buf);
 		result = xstrdup("");
-	} else
+	} else {
+		strbuf_trim(&buf);
 		result = strbuf_detach(&buf, NULL);
+	}
 
 	strbuf_release(&cmd);
 	return result;