patch 8.2.1398: autoload script sourced twice if sourced directly

Problem:    Autoload script sourced twice if sourced directly.
Solution:   Do not source an autoload script again. (issue #6644)
diff --git a/src/testdir/sautest/autoload/sourced.vim b/src/testdir/sautest/autoload/sourced.vim
index f69f00c..aac96b1 100644
--- a/src/testdir/sautest/autoload/sourced.vim
+++ b/src/testdir/sautest/autoload/sourced.vim
@@ -1,3 +1,4 @@
 let g:loaded_sourced_vim += 1
-func! sourced#something()
+func sourced#something()
 endfunc
+call sourced#something()