Fetch all branches in GitHub action

[int-y1]

origin/update-i18n wasn't fetched, so the action went with i18n_head=HEAD. To fix this, fetch all branches.

[codecov-commenter]

Codecov Report

Merging #1885 (bef1dc7) into master (9d29b04) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1885   +/-   ##
=======================================
  Coverage   46.24%   46.24%           
=======================================
  Files         231      231           
  Lines       13054    13054           
=======================================
  Hits         6037     6037           
  Misses       7017     7017           

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[Ninjaclasher]

LGTM

[quantum5]

Only one way to find out. I suspect this is not the end of it: it'll probably close the PR if no file has material change. I think you know what to do in that case: if no file went down the changed path, git reset --hard "$i18n_head".