hash-object: merge HASH_* and INDEX_* flags

hash-object: merge HASH_* and INDEX_* flags

The hash-object command has its own custom flag bits that it sets based
on command-line options. But since we dropped hash_literally() in the
previous commit, the only thing we do with those flag bits is convert
them directly into "index_flags" to pass to index_fd().

This extra layer of indirection makes the code harder to read and reason
about. Let's just use the INDEX_* flags directly.

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