docs: describe impact of repack on "clone -s"

docs: describe impact of repack on "clone -s"

The effects of repacking on a repository with alternates are a bit
subtle. The two main things users will want are:

  1. Not to waste disk space by accidentally copying objects which could
     be shared.

  2. Copying all objects explicitly to break the dependency on the source
     repo.

This patch describes both under the "clone -s" documentation. It makes
sense to put it there rather than in git-repack.txt for both cases.

For (1), we are warning the user who is using "clone -s" about what _not_
to do, so we need to get their attention when reading about "clone -s".

For (2), we are telling them how git-repack can be used to accomplish a
task, but until they know that git-repack is the right tool, they have no
reason to look at the repack documentation.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>