RFR: 1995: The logic for getting repository name and branch name in the backport pr command is inconsistent

[Zhao Song]

In the backport pull request command, the process of getting the repository name and branch name differs between cancelling a backport request and creating a backport request. 

In this patch, I made the logic consistent.

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Commit messages:
 - SKARA-1995

Changes: https://git.openjdk.org/skara/pull/1545/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1545&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1995
  Stats: 21 lines in 1 file changed: 10 ins; 1 del; 10 mod
  Patch: https://git.openjdk.org/skara/pull/1545.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1545/head:pull/1545

PR: https://git.openjdk.org/skara/pull/1545