fast-import: make default pack size unlimited

fast-import: make default pack size unlimited

Now that fast-import is creating packs with index version 2, there is
no point limiting the pack size by default.  A pack split will still
happen if off_t is not sufficiently large to hold large offsets.

While updating the doc, let's remove the "packfiles fit on CDs"
suggestion.  Pack files created by fast-import are still suboptimal and
a 'git repack -a -f -d' or even 'git gc --aggressive' would be a pretty
good idea before considering storage on CDs.

Signed-off-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>