setup: update the right file in multiple checkouts

This code is introduced in 23af91d (prune: strategies for linked
checkouts - 2014-11-30), and it's supposed to implement this rule from
that commit's message:

 - linked checkouts are supposed to keep its location in $R/gitdir up
   to date. The use case is auto fixup after a manual checkout move.

Note the name, "$R/gitdir", not "$R/gitfile". Correct the path to be
updated accordingly.

While at there, make sure I/O errors are not silently dropped.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

setup.c

@@ -402,9 +402,9 @@ static void update_linked_gitdir(const char *gitfile, const char *gitdir)
 	struct strbuf path = STRBUF_INIT;
 	struct stat st;
 
-	strbuf_addf(&path, "%s/gitfile", gitdir);
+	strbuf_addf(&path, "%s/gitdir", gitdir);
 	if (stat(path.buf, &st) || st.st_mtime + 24 * 3600 < time(NULL))
-		write_file(path.buf, 0, "%s\n", gitfile);
+		write_file(path.buf, 1, "%s\n", gitfile);
 	strbuf_release(&path);
 }