Speed up package--list-loaded-files a bit

Fixes: debbugs:19390

* lisp/emacs-lisp/package.el (package--list-loaded-files): Don't call
file-truename on load-history elements.
* lisp/vc/vc.el: Improve `dir-status-files' description
* calc.texi (About This Manual): Update instructions
for building the manual.