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

[Zhao Song]

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.

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Commit messages:
 - SKARA-1849

Changes: https://git.openjdk.org/skara/pull/1488/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1488&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1849
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/skara/pull/1488.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1488/head:pull/1488

PR: https://git.openjdk.org/skara/pull/1488