Implemented the core of the merge-file and per-dir merge-file support

for the --filter option.  Some noteworthy changes:
- Added push_local_excludes() and pop_local_excludes() to implement
  the changes needed as we go from directory to directory (this takes
  the place of the old local_exclude_list var that only handled the
  .cvsignore files).  These per-dir excludes are linked into the same
  exclude list as the global excludes.
- The exclude list is transferred as filter rules to an rsync that
  talks protocol 29 or above.  It limits itself to just include/exclude
  rules for older rsyncs.