Fixes for emacs-repository-get-version

Fixes for emacs-repository-get-version

* lisp/version.el (emacs-repository-get-version):
Check either .bzr or .git, but not both.
Make the git case actually use the DIR argument, and return nil
rather than the empty string.

* etc/NEWS: Don't mention that emacs-repository-version works for git,
because it only returns nil (since it uses %N and there seem to be no
notes in the git version of the Emacs repo.)
No idea why this change is in trunk during a feature freeze anyway.