fix(filter): accept e/n/w/- merge modifiers on merge/dir-merge rules

The earlier modifier-rejection change rejected e/n/w on all rules, but rsync
3.4.1 accepts them (plus the '-' merge-only modifier) on merge and dir-merge
rules.  Restrict the rejection to non-merge rules and consume the merge-file
modifiers (e/n/w/-) so they no longer leak into the merge filename.

- is_merge_rule()/is_merge_modifier_char() gate the merge-only modifiers.
- scan vs consume sets: e/n/w still count as modifier-run chars on every rule
  (pure tokens like -new/-press stay rejected), but are only consumed on merge
  rules, preserving mixed-token parsing such as H,!secret -> ecret.
- '-' is accepted/consumed only on merge/dir-merge (e.g. dir-merge,- .rules).
- x remains rejected everywhere with its dedicated message.
- e/n/w/- semantics remain unimplemented and are documented as accepted-but-
  ignored in filter.h.

Tests: split the merge forms out of the rejection test into a new acceptance
test asserting the merge file is read and dir_merge_names keeps the modifier-
free basename; non-merge pure-modifier forms still rejected.

src/shared/filter.h

@@ -24,8 +24,12 @@
  *   clear/!     clear the current rule list (takes no argument)
  * Modifiers: '/' absolute anchor, '!' negate match, 'C' inject CVS defaults,
  * 's' sender side, 'r' receiver side, 'p' perishable.  The rsync 'x'
- * (xattr-name) modifier and the merge-only 'e'/'n'/'w' modifiers are not
- * implemented and are rejected explicitly.
+ * (xattr-name) modifier is not implemented and is rejected explicitly
+ * everywhere.  The merge-file modifiers 'e' (exclude the merge file itself),
+ * 'n' (do not inherit the merge file), 'w' (word-split the merge file) and '-'
+ * (do not transfer the merge file) are accepted and consumed only on merge/
+ * dir-merge rules (rejected on every other rule, matching rsync); their
+ * semantics are not implemented and they are otherwise ignored.
  * A trailing '/' makes a pattern match directories only.  A leading '/' anchors
  * the pattern to its owner directory.
  */