fast-import: increase the default pack depth to 50

fast-import: increase the default pack depth to 50

In 618e613a70, 10 years ago, the default for pack depth used for
git-pack-objects and git-repack was changed from 10 to 50, while
leaving fast-import's default to 10.

There doesn't seem to be a reason besides oversight for the change not
having happened in fast-import as well.

Interestingly, fast-import uses pack.depth when it's set, and the
git-config manual says the default for pack.depth is 50. While the
git-fast-import manual does say the default depth is 10, the
inconsistency is also confusing.

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>