Merge branch 'ds/reprepare-alternates-when-repreparing-packfiles'

Once we start running, we assumed that the list of alternate object
databases would never change.  Hook into the machinery used to
update the list of packfiles during runtime to update this list as
well.

* ds/reprepare-alternates-when-repreparing-packfiles:
  object-file: reprepare alternates when necessary

diff --cc packfile.c
Simple merge