read-cache.c: read prefix-compressed names in index on-disk version v4

read-cache.c: read prefix-compressed names in index on-disk version v4

Because the entries are sorted by path, adjacent entries in the index tend
to share the leading components of them, and it makes sense to only store
the differences in later entries.  In the v4 on-disk format of the index,
each on-disk cache entry stores the number of bytes to be stripped from
the end of the previous name, and the bytes to append to the result, to
come up with its name.

Signed-off-by: Junio C Hamano <gitster@pobox.com>