sha1sum: Fix hashing for data >= 256 GiB
* src/sha1sum.c (DIGEST_CONTEXT): Add nblocks_high. (digest_init): Clear that var. (digest_write): Bump it up on overflow of nblocks. (digest_final): Stuff it into the MSB.
See also GnuPG commit c43a8c0d81a711161f7a81b24ef7c33a1353eee0
dated 2013-11-14.
- GnuPG-bug-id: T7592