git post-receive hook to trigger drush rsync without prompt or with predefined answer
Add the --yes option to your Drush command.  This assuredly will work if you place it before the rsync directive.

For example:

drush --yes rsync @dev @live


Drush rsync is different than most Drush commands in that it uses strict option handling.  Commands that work with strict options pass all extra flags that appear after the Drush command name on to the tool that is being invoked.  Thus, drush rsync --yes @dev @live will fail, because the --yes will be ignored by Drush and passed on to rsync.

For more information, see:

drush topic docs-strict-options