fsck: check index files in all worktrees

fsck: check index files in all worktrees

We check the index file for the main worktree, but completely ignore the
index files in other worktrees. These should be checked, too, as they
are part of the repository state (and in particular, errors in those
index files may cause repo-wide operations like "git gc" to complain).

Reported-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>