Don't prompt unnecessarily when doing C-c C-t in a Muse project file.

Don't prompt unnecessarily when doing C-c C-t in a Muse project file.

* lisp/muse-publish.el (muse-publish-get-output-dir): Use
  `muse-read-directory-name' instead of `read-file-name'.  Now Muse won't
  offer the style and directory prompt when you hit C-c C-t in a file
  belonging to a defined Muse project.  Otherwise prompt.  Thanks to Yann
  Hodique for the fix.

* lisp/muse.el (muse-read-directory-name): New function that defines a
  fallback version of `read-directory-name', since Emacs21 doesn't have
  it.
git-archimport-id: mwolson@gnu.org--2005/muse--main--1.0--patch-209