read-cache: use get_be32 instead of hand-rolled ntoh_l

read-cache: use get_be32 instead of hand-rolled ntoh_l

Commit d60c49c (read-cache.c: allow unaligned mapping of the
index file, 2012-04-03) introduced helpers to access
unaligned data. However, we already have get_be32, which has
a few advantages:

  1. It's already written, so we avoid duplication.

  2. It's probably faster, since it does the endian
     conversion and the alignment fix at the same time.

  3. The get_be32 code is well-tested, having been in
     block-sha1 for a long time. By contrast, our custom
     helpers were probably almost never used, since the user
     needed to manually define a macro to enable them.

We have to add a get_be16 implementation to the existing
get_be32, but that is very simple to do.

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