send-pack: switch to using git-rev-list --stdin

send-pack: switch to using git-rev-list --stdin

When we are generating packs to update remote repositories we
want to supply as much information as possible about the revisions
that already exist to rev-list in order optimise the pack as much
as possible.  We need to pass two revisions for each branch we are
updating in the remote repository and one for each additional branch.
Where the remote repository has numerous branches we can run out
of command line space to pass them.

Utilise the git-rev-list --stdin mode to allow unlimited numbers
of revision constraints.  This allows us to move back to the much
simpler unordered revision selection code.

[jc: added some comments in the code to describe the pipe flow
 a bit.]

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>