submodule: use capture_command

In is_submodule_commit_present, we call run_command followed
by a pipe read, which is prone to deadlock. It is unlikely
to happen in this case, as rev-list should never produce
more than a single line of output, but it does not hurt to
avoid an anti-pattern (and using the helper simplifies the
setup and cleanup).

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

submodule.c

@@ -576,12 +576,10 @@ static int is_submodule_commit_present(const char *path, unsigned char sha1[20])
 		cp.env = local_repo_env;
 		cp.git_cmd = 1;
 		cp.no_stdin = 1;
-		cp.out = -1;
 		cp.dir = path;
-		if (!run_command(&cp) && !strbuf_read(&buf, cp.out, 1024))
+		if (!capture_command(&cp, &buf, 1024) && !buf.len)
 			is_present = 1;
 
-		close(cp.out);
 		strbuf_release(&buf);
 	}
 	return is_present;