Version control post-commit hooks and Trac
Up to Trac 0.11, Trac was distributed with a post-commit hook for Subversion which could be used to automatically modify tickets based on the content of the commit log.
However, this was quite error prone (e.g. #10458) and that was replaced by a more general mechanism based on a changeset notifications interface and normal plugins reacting on these notifications. See CommitTicketUpdater.
Now the source:trunk/contrib/trac-svn-hook is just used to trigger a notification signal via the trac-admin, nothing more. The script is not necessary and trac-admin can be called directly in your post-commit hook.
For version control systems other than Subversion, see also TracRepositoryAdmin#ExplicitSync.