Clean up merge list handling:

Clean up merge list handling:
* In depgraph.loadResumeCommand(), populate _serialized_tasks_cache so that
  it can be returned from depgraph.altlist().
* Handle --skipfirst instide depgraph.loadResumeCommand(), and fix it to skip
  the first "merge" task and never skip "uninstall" tasks.
* Always get the merge list from depgraph.altlist() instead of using
  the mtimedb directly. The list returned from depgraph.altlist()
  contains Package instances with cached metadata.
* Remove obsolete code from depgraph.display() and MergeTask.merge() since
  the merge list always comes from depgraph.altlist() now, and never directly
  from the mtimedb.

svn path=/main/trunk/; revision=10092