RFR: 1133: Source branch for /backport PRs is outdated [v3]

[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.

Kevin Rushforth has updated the pull request incrementally with one additional commit since the last revision:

  Address review coments

-------------

Changes:
  - all: https://git.openjdk.java.net/skara/pull/1209/files
  - new: https://git.openjdk.java.net/skara/pull/1209/files/8cb4244d..4f24176d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=1209&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=1209&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 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