improve fetch-pack's handling of kept packs

improve fetch-pack's handling of kept packs

Since functions in fetch-clone.c were only used from fetch-pack.c,
its content has been merged with fetch-pack.c.  This allows for better
coupling of features with much simpler implementations.

One new thing is that the (abscence of) --thin also enforce it on
index-pack now, such that index-pack will abort if a thin pack was
_not_ asked for.

The -k or --keep, when provided twice, now causes the fetched pack
to be left as a kept pack just like receive-pack currently does.
Eventually this will be used to close a race against concurrent
repacking.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>