RFR: 1849: Exception when creating a new PR from CLI

[Erik Joelsson]

On Mon, 20 Mar 2023 18:25:32 GMT, Zhao Song <zsong at openjdk.org> wrote:

> Since `git-for-each-ref` is updated in git 2.40.0, the output of this command is changed and would invoke some errors when using `git pr create`.
> 
> In this patch, we would filter out invalid output when trying to find candidate target refs.

Marked as reviewed by erikj (Lead).

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PR Review: https://git.openjdk.org/skara/pull/1488#pullrequestreview-1349248427