(#18494) Rearrange loop to remove Ruby 2.0 warning

Ruby 2.0 warned of the complex assignments in the while loop:

  lib/puppet/file_serving/fileset.rb:139: warning: found = in conditional, should be ==

The block that shifts to the next recursion level has been removed from the
loop expression to prevent the interpreter from warning.

lib/puppet/file_serving/fileset.rb

@@ -136,7 +136,7 @@ def perform_recursion
     result = []
     return result unless recurse?(depth)
 
-    while dir_path = current_dirs.shift or ((depth += 1) and recurse?(depth) and current_dirs = next_dirs and next_dirs = [] and dir_path = current_dirs.shift)
+    while dir_path = current_dirs.shift
       next unless stat = stat(dir_path)
       next unless stat.directory?
 
@@ -153,6 +153,14 @@ def perform_recursion
         # And to our list of files/directories to iterate over.
         next_dirs << File.join(dir_path, file_path)
       end
+
+      # Move to the next recusion level
+      if current_dirs.empty?
+        depth += 1
+        break unless recurse?(depth)
+        current_dirs = next_dirs
+        next_dirs = []
+      end
     end
 
     result