builtin/repack.c: parse `--max-pack-size` with OPT_MAGNITUDE

builtin/repack.c: parse `--max-pack-size` with OPT_MAGNITUDE

The repack builtin takes a `--max-pack-size` command-line argument which
it uses to feed into any of the pack-objects children that it may spawn
when generating a new pack.

This option is parsed with OPT_STRING, meaning that we'll accept
anything as input, punting on more fine-grained validation until we get
down into pack-objects.

This is fine, but it's wasteful to spend an entire sub-process just to
figure out that one of its option is bogus. Instead, parse the value of
`--max-pack-size` with OPT_MAGNITUDE in 'git repack', and then pass the
known-good result down to pack-objects.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>