RFR: 8310574 GenShen: Should not update-references for in-place-promotions [v3]

[William Kemper]

> When a cycle _only_ promotes regions in place, it is not necessary to update refs.

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:

 - Merge branch 'shenandoah-master' into skip-update-refs-v2
 - Bring formatting up to standard
 - Skip update-refs when collection set is empty but regions were promoted in place

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Changes: https://git.openjdk.org/shenandoah/pull/291/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=291&range=02
  Stats: 46 lines in 4 files changed: 30 ins; 10 del; 6 mod
  Patch: https://git.openjdk.org/shenandoah/pull/291.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/291/head:pull/291

PR: https://git.openjdk.org/shenandoah/pull/291