- Fixed the handling of the "!" list-clearing rules.

- Fixed the handling of the "!" list-clearing rules.
- Fixed a potential crash bug in the clearing of a per-dir
  merge file via "!".
- Simplified the push/pop structure for per-dir merge files.
- Some minor enhancements to exclude.test.
- Improved the parsing of filters rules when the "s" modifier is
  used (where rules are split on whitespace).
- Allow '_' to separate the prefix from the suffix in a filter rule.
- Improved some debug messages relating to includes/excludes.