RFR: 1133: Source branch for /backport PRs is outdated

[Kevin Rushforth]

As mentioned in the JBS bug report for this bug, [SKARA-1080](https://bugs.openjdk.java.net/browse/SKARA-1080), which was fixed by PR #1188, was only a partial fix of the problem where the `/backport` command creates the local branch for the PR based off a stale copy of the target branch. The solution is to fetch the target branch from the target repo and base the patch off of that branch.

I didn't add any new tests for this case, so I'll file a follow-up issue.

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Commit messages:
 - 1133: Source branch for /backport PRs is outdated

Changes: https://git.openjdk.java.net/skara/pull/1209/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1209&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1133
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/1209.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1209/head:pull/1209

PR: https://git.openjdk.java.net/skara/pull/1209