tests: add line specific for testing with fsmonitor

tests/test-hgignore.t

@@ -178,12 +178,17 @@ Test relative ignore path (issue4473):
 
 Check patterns that match only the directory
 
+"(fsmonitor !)" below assumes that fsmonitor is enabled with
+"walk_on_invalidate = false" (default), which doesn't involve
+re-walking whole repository at detection of .hgignore change.
+
   $ echo "^dir\$" > .hgignore
   $ hg status
   A dir/b.o
   ? .hgignore
   ? a.c
   ? a.o
+  ? dir/c.o (fsmonitor !)
   ? syntax
 
 Check recursive glob pattern matches no directories (dir/**/c.o matches dir/c.o)