RFR: Only consider backports with a proper link relationship

[Robin Westberg]

If an issue links to a backport, it should only be consider an actual backport if the link type is correct (to avoid looking at backport issues linked as "related to" for example).

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

Commit messages:
 - Only consider backports with a proper link relationship

Changes: https://git.openjdk.java.net/skara/pull/960/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=960&range=00
  Stats: 12 lines in 2 files changed: 12 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/960.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/960/head:pull/960

PR: https://git.openjdk.java.net/skara/pull/960