blame.c: prepare_lines should not call xrealloc for every line

Making a single preparation run for counting the lines will avoid memory
fragmentation.  Also, fix the allocated memory size which was wrong
when sizeof(int *) != sizeof(int), and would have been too small
for sizeof(int *) < sizeof(int), admittedly unlikely.

Signed-off-by: David Kastrup <dak@gnu.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>