emacs: fix changelog URL for mainline

- Point URL to savannah’s cgit, the canonical source.
- Select tag instead of release branch, which is more precise.

Compare the differences:
bad (branch, intermediate state pre-30.3):
https://www.gnu.org/savannah-checkouts/gnu/emacs/news/NEWS.30.2

good (tag, precisely the state we are shipping):
https://cgit.git.savannah.gnu.org/cgit/emacs.git/plain/etc/NEWS?h=emacs-30.2

pkgs/applications/editors/emacs/sources.nix

@@ -74,7 +74,7 @@ let
         '';
         changelog =
           {
-            "mainline" = "https://www.gnu.org/savannah-checkouts/gnu/emacs/news/NEWS.${version}";
+            "mainline" = "https://cgit.git.savannah.gnu.org/cgit/emacs.git/plain/etc/NEWS?h=${rev}";
             "macport" = "https://github.com/jdtsmith/emacs-mac/blob/${rev}/NEWS-mac";
           }
           .${variant};