Update .gitignore with the new location for themes directory (#39111).

git-svn-id: https://svn.redmine.org/redmine/trunk@22712 e93f8b46-1217-0410-a6f0-8f06a7374b81

.gitignore

@@ -27,10 +27,8 @@
 /public/assets/*
 /public/dispatch.*
 /public/plugin_assets/*
-/public/themes/*
-!/public/themes/alternate
-!/public/themes/classic
-!/public/themes/README
+/themes/*
+!/themes/README
 /tmp/*
 /tmp/cache/*
 /tmp/pdf/*