dir: support platforms that require aligned reads

The untracked cache is stored on disk by concatenating its memory
structures without any padding.  Consequently some of the structs are
not aligned at a particular boundary when the whole extension is read
back in one go.  That's only OK on platforms without strict alignment
requirements, or for byte-aligned data like strings or hash values.

Decode struct ondisk_untracked_cache carefully from the extension
blob by using explicit pointer arithmetic with offsets, avoiding
alignment issues.  Use char pointers for passing stat_data objects to
stat_data_from_disk(), and use memcpy(3) in that function to  get the
contents into a properly aligned struct, then perform the byte-order
adjustment in place there.

Found with Clang's UBSan.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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