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.