fetch-pack: remove --keep-auto and make it the default.

fetch-pack: remove --keep-auto and make it the default.

This makes git-fetch over git native protocol to automatically
decide to keep the downloaded pack if the fetch results in more
than 100 objects, just like receive-pack invoked by git-push
does.  This logic is disabled when --keep is explicitly given
from the command line, so that a very small clone still keeps
the downloaded pack as before.

The 100 threshold can be adjusted with fetch.unpacklimit
configuration.  We might want to introduce transfer.unpacklimit
to consolidate the two unpacklimit variables, which will be a
topic for the next patch.

Signed-off-by: Junio C Hamano <junkio@cox.net>