Add tmp/ to generated plugin .gitignore

The documentation for Rails::Generators::TestCase suggests using tmp/ as
the destination directory when testing generators.  Many plugins test
generators, so it makes sense to include tmp/ in the default .gitignore.

railties/lib/rails/generators/rails/plugin/templates/gitignore.tt

@@ -1,6 +1,7 @@
 /.bundle/
 /log/*.log
 /pkg/
+/tmp/
 <% if with_dummy_app? -%>
 <% if sqlite3? -%>
 /<%= dummy_path %>/db/*.sqlite3