RFR: 1080: /backport command reports conflict even though there isn't any

[Erik Joelsson]

This patch fixes a bug with the /backport commit command. It currently materializes the workspace where the cherry-picking is done from the wrong repository. It should be using the target repository, but instead it's using the "fork" (which is the repo from which we create the pull request if /backport is successful). 

In the tests, this wasn't handled at all. All the tests used the same hosted repository for the "fork" as the target. I introduced a separate "fork" repository in the tests where it would make a difference. 

I changed the backportDoesNotApply test so that it would fail without this fix. I also think I made it clearer why the backport shouldn't apply as well.

Finally I added a new test which verifies a situation very similar to the original issue reported.

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Depends on: https://git.openjdk.java.net/skara/pull/1187

Commit messages:
 - SKARA-1080

Changes: https://git.openjdk.java.net/skara/pull/1188/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1188&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1080
  Stats: 88 lines in 2 files changed: 81 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/skara/pull/1188.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1188/head:pull/1188

PR: https://git.openjdk.java.net/skara/pull/1188