fsck: factor out index fsck

fsck: factor out index fsck

The code to fsck an index operates directly on the_index. Let's move it
into its own function in preparation for handling the index files from
other worktrees.

Since we now have only a single reference to the_index, let's drop
our USE_THE_INDEX_VARIABLE definition and just use the_repository.index
directly. That's a minor cleanup, but also ensures that we didn't miss
any references when moving the code into fsck_index().

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