diffcore-delta: avoid ignoring final 'line' of file

hash_chars() would hash lines to integers, and store them in a spanhash,
but cut lines at 64 characters.  Thus, whenever it reached 64 characters
or a newline, it would create a new spanhash.  The problem is, the final
part of the file might not end 64 characters after the previous 'line'
and might not end with a newline.  This could, for example, cause an
85-byte file with 12 lines and only the first character in the file
differing to appear merely 23% similar rather than the expected 97%.
Ensure the last line is included, and add a testcase that would have
caught this problem.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diffcore-delta.c

@@ -159,6 +159,10 @@ static struct spanhash_top *hash_chars(struct repository *r,
 		n = 0;
 		accum1 = accum2 = 0;
 	}
+	if (n > 0) {
+		hashval = (accum1 + accum2 * 0x61) % HASHBASE;
+		hash = add_spanhash(hash, hashval, n);
+	}
 	QSORT(hash->data, (size_t)1ul << hash->alloc_log2, spanhash_cmp);
 	return hash;
 }