RFR: update-sync labels should be applied to all-bar-one records

[Rob McKenna]

This modification fixes the situation where a backport record may be a member
of two distinct streams and has its hgupdate-sync label removed as a result.

E.g. with backport records for 14, 15, 14.0.1, 15.0.1 no label would be
applied to 15. The current logic gives precedence to the fact that it is the
first release in the 15-updates stream and ignores the fact that it is not
the first release in the feature stream. The label added by the feature stream
is removed when evaluating the 15-updates stream. This should not be the case.

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Commit messages:
 - update-sync labels should be applied to all bar one record

Changes: https://git.openjdk.java.net/skara/pull/848/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=848&range=00
  Stats: 35 lines in 2 files changed: 27 ins; 1 del; 7 mod
  Patch: https://git.openjdk.java.net/skara/pull/848.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/848/head:pull/848

PR: https://git.openjdk.java.net/skara/pull/848