feat: fetch for default branch

[yoshi-automation]

This PR was generated using Autosynth. :rainbow:

Synth log will be available here: https://source.cloud.google.com/results/invocations/0f7d18dd-0fed-4284-8748-9a058bdad02f/targets

• [ ] To automatically regenerate this PR, check this box. (May take up to 24 hours.)

Source-Link: https://github.com/googleapis/synthtool/commit/898b38a6f4fab89a76dfb152480bb034a781331b