RFR: 1140: Add test case for stale backport branches

[Guoxiong Li]

Hi all,

This patch adds the test cases about fetching the target branch when using the backport command each time.

Thanks for taking the time to review.

Best Regards,
-- Guoxiong

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Commit messages:
 - 1140: Add test case for stale backport branches

Changes: https://git.openjdk.java.net/skara/pull/1253/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1253&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1140
  Stats: 21 lines in 1 file changed: 20 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/1253.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1253/head:pull/1253

PR: https://git.openjdk.java.net/skara/pull/1253