Teach "git clone" to pack refs

Teach "git clone" to pack refs

In repos with many refs, it is unlikely that most refs will ever change.
This fact is already exploited by "git gc" by executing "git pack-refs"
to consolidate all refs into a single file.

When cloning a repo with many refs, it does not make sense to create the
loose refs in the first place, just to have the next "git gc" consolidate
them into one file. Instead, make "git clone" create the packed refs file
immediately, and forego the loose refs completely.

Signed-off-by: Johan Herland <johan@herland.net>
Acked-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>