Merge branch 'refactor-tmp-gitignore' into 'master'

Speed up git status by refactoring gitignore

See merge request gitlab-org/gitlab!51681

tmp/.gitignore

 *
-!*/
 !.gitignore
 !.gitkeep
+
+# explicitly list ignored sub directories to speed up performance
+/cache/
+/capybara/
+/letter_opener/
+/rubocop_cache/
 /tests/