Update author generation script to retain existing ones.

tools/generate-authors.fish

@@ -5,8 +5,13 @@ echo '# This file lists all individuals having contributed content to the reposi
 echo '# This list was auto-generated from Git history.'
 echo
 
-git log --format='%aN <%aE>' | awk '
-{
+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, "<");
         name = substr($0, 0, pos - 2);
         email = substr($0, pos + 1, length($0) - pos - 1);
@@ -15,5 +20,8 @@ git log --format='%aN <%aE>' | awk '
         if (names[name] == 1 && emails[email] == 1) {
             print $0;
         }
-}
-' | env LC_ALL=C.UTF-8 sort -uf
+    }
+    '
+end
+
+list_authors | env LC_ALL=C.UTF-8 sort -uf