fast-import: count --max-pack-size in bytes

fast-import: count --max-pack-size in bytes

Similar in spirit to 07cf0f2 (make --max-pack-size argument to 'git
pack-object' count in bytes, 2010-02-03) which made the option by the same
name to pack-objects, this counts the pack size limit in bytes.

In order not to cause havoc with people used to the previous megabyte
scale an integer smaller than 8192 is interpreted in megabytes but the
user gets a warning.  Also a minimum size of 1 MiB is enforced to avoid an
explosion of pack files.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Acked-by: Nicolas Pitre <nico@fluxnic.net>