index-pack, unpack-objects: use get_be32() for reading pack header

index-pack, unpack-objects: use get_be32() for reading pack header

Both of these commands read the incoming pack into a static unsigned
char buffer in BSS, and then parse it by casting the start of the buffer
to a struct pack_header. This can result in SIGBUS on some platforms if
the compiler doesn't place the buffer in a position that is properly
aligned for 4-byte integers.

This reportedly happens with unpack-objects (but not index-pack) on
sparc64 when compiled with clang (but not gcc). But we are definitely in
the wrong in both spots; since the buffer's type is unsigned char, we
can't depend on larger alignment. When it works it is only because we
are lucky.

We'll fix this by switching to get_be32() to read the headers (just like
the last few commits similarly switched us to put_be32() for writing
into the same buffer).

It would be nice to factor this out into a common helper function, but
the interface ends up quite awkward. Either the caller needs to hardcode
how many bytes we'll need, or it needs to pass us its fill()/use()
functions as pointers. So I've just fixed both spots in the same way;
this is not code that is likely to be repeated a third time (most of the
pack reading code uses an mmap'd buffer, which should be properly
aligned).

I did make one tweak to the shared code: our pack_version_ok() macro
expects us to pass the big-endian value we'd get by casting. We can
introduce a "native" variant which uses the host integer ordering.

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