feat: fetch for default branch

[yoshi-automation]

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

[codecov[bot]]

Codecov Report

Merging #238 (69967f1) into master (29abfd0) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #238   +/-   ##
=======================================
  Coverage   79.64%   79.64%           
=======================================
  Files           3        3           
  Lines         113      113           
  Branches       13       14    +1     
=======================================
  Hits           90       90           
  Misses         23       23           

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