Update author generation script to retain existing ones.

tools/generate-authors.fish

@@ -5,6 +5,11 @@ echo '# This file lists all individuals having contributed content to the reposi
 echo '# This list was auto-generated from Git history.'
 echo
 
+function list_authors
+    # List existing authors
+    git show HEAD:AUTHORS | sed -e '/^$/d' -e '/^#/d'
+
+    # List any new authors
     git log --format='%aN <%aE>' | awk '
     {
         pos = index($0, "<");
@@ -16,4 +21,7 @@ git log --format='%aN <%aE>' | awk '
             print $0;
         }
     }
-' | env LC_ALL=C.UTF-8 sort -uf
+    '
+end
+
+list_authors | env LC_ALL=C.UTF-8 sort -uf