make --max-pack-size argument to 'git pack-object' count in bytes

make --max-pack-size argument to 'git pack-object' count in bytes

The value passed to --max-pack-size used to count in MiB which was
inconsistent with the corresponding configuration variable as well as
other command arguments which are defined to count in bytes with an
optional unit suffix.  This brings --max-pack-size in line with the
rest of Git.

Also, in order not to cause havoc with people used to the previous
megabyte scale, and because this is a sane thing to do anyway, a
minimum size of 1 MiB is enforced to avoid an explosion of pack files.

Adjust and extend test suite accordingly.

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