repack: rewrite the shell script in C

repack: rewrite the shell script in C

The motivation of this patch is to get closer to a goal of being
able to have a core subset of git functionality built in to git.
That would mean

 * people on Windows could get a copy of at least the core parts
   of Git without having to install a Unix-style shell

 * people using git in on servers with chrooted environments
   do not need to worry about standard tools lacking for shell
   scripts.

This patch is meant to be mostly a literal translation of the
git-repack script; the intent is that later patches would start using
more library facilities, but this patch is meant to be as close to a
no-op as possible so it doesn't do that kind of thing.

Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>