x/review/git-codereview: Set git config commit.gpgsign to false

[hnakamur]

Otherwise if commit.gpgsign is true in ~/.gitconfig, the following error occurs:

error: There was a problem with the editor 'false'. Please supply the message using either -m or -F option.

Fixes #67402

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

This PR (HEAD: 8269fee) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/review/+/585875.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to log in to Gerrit.
• To change your code in response to feedback:
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• The title and description of the GitHub PR are used to construct the final commit message.
  • Edit these as needed via the GitHub web interface (not via Gerrit or git).
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