Tab completion for git branches showing old/outdated entries

I'm on a sort of frankendebian stretch/sid (not the best idea, I know; planning on reinstalling soon).

Tab completion works for git branch names in git repo directories:

:~/project $ git checkout <TAB><TAB>
Display all 200 possibilities? (y or n)

:~/project $ git checkout private-rl_<TAB><TAB>
private-rl_1219_misspelled_locale_zhtw   private-rl_1950_scheduler_offset         private-rl_bootstrap_rake_tasks
private-rl_1854_ldap_filter_reset        private-rl_bootstrap_rake_task

But some of the branches it shows don't exist anymore:

:~/project $ git branch
* develop
  private-rl_1219_misspelled_locale_zhtw
  stable

This also happens for deleted remote branches.

What's going on here? Does the git completion script keep a cache of old branches that can be flushed somehow? How can I stop these branches from accumulating in my tab-completion results?