Replication steps:
As admin, set defaulthomepage to "User preference".
Log in as normal user with Shibboleth authentication and check start page.

Expected result:
When defaulthomepage is set to "User preference" (value="2"), users logging in via Shibboleth should be redirected to their start page as set in user preferences.

Actual result:
Users logging in via Shibboleth are redirected to Moodle's home page regardless of their prefererences.

This could be fixed by setting $urltogo in auth/shibboleth/index.php according to Moodle's function get_home_page instead of $CFG->defaulthomepage.