[PATCH] Don't fetch objects that exist in the local repository

[PATCH] Don't fetch objects that exist in the local repository

Be sure not to fetch objects that already exist in the local repository.
The main process loop no longer performs this check, http-fetch now checks
prior to starting a new request queue entry and when fetch_object() is called,
and local-fetch now checks when fetch_object() is called.

As discussed in this thread: http://marc.theaimsgroup.com/?t=112854890500001

Signed-off-by: Nick Hengeveld <nickh@reactrix.com>