If any file changed only parse all files if force-update is true

Commit 13e9a44 enforced parsing all
files if any file is newer than the previous parse, not only updated
files.

However, this did not take into account the `--force-update` option and
would parse all files even if `--force-update` was false.

After this commit, if `--force-update` is false, only files newer than
the previous parse will be parsed.

lib/rdoc/rdoc.rb

@@ -125,12 +125,14 @@ def gather_files files
 
     file_list = remove_unparseable(file_list)
 
+    result = []
     if file_list.count {|name, mtime|
+         result << name if mtime || @options.force_update
          file_list[name] = @last_modified[name] unless mtime
          mtime
        } > 0
       @last_modified.replace file_list
-      file_list.keys.sort
+      result.sort
     else
       []
     end

test/rdoc/test_rdoc_rdoc.rb

@@ -80,6 +80,17 @@ def test_gather_files
     assert_equal [a, b], @rdoc.gather_files([b, a, b])
   end
 
+  def test_gather_files_with_no_force_update
+    a = File.expand_path __FILE__
+    b = File.expand_path '../test_rdoc_text.rb', __FILE__
+
+    assert_equal [a, b], @rdoc.gather_files([a, b])
+
+    @rdoc.last_modified[a] -= 10
+    @rdoc.options.force_update = false
+    assert_equal [a], @rdoc.gather_files([a, b])
+  end
+
   def test_handle_pipe
     $stdin = StringIO.new "hello"