RFR: 1345: sync-bot removing hgupdate-sync label from 8u5 backport

[Erik Joelsson]

This patch fixes an issue in the synclabel bot. For update releases of 8 and 7, we have special ranges of build numbers that are used for special releases. These need to be, and are, handled separately in the synclabel bot. These ranges only apply to update releases however, and not the main feature releases, where the actual build numbers used went well into these ranges. 

Added a test that mimics the example mislabeled set of bugs.

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Commit messages:
 - SKARA-1345

Changes: https://git.openjdk.java.net/skara/pull/1284/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1284&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1345
  Stats: 15 lines in 2 files changed: 14 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/1284.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1284/head:pull/1284

PR: https://git.openjdk.java.net/skara/pull/1284