RFR: 8365026: G1: Initialization should start a "full" new collection set

[Thomas Schatzl]

Hi all,

  please review this change that makes G1 start a "full" new collection set at initialization time, calling `G1CollectedHeap::start_new_collection_set()`.

Previously G1 only did part of that (start incremental building only), on a not-fully initialized collection set instance. It works, but is not nice.

Testing: gha

Thanks,
  Thomas

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Commit messages:
 - 8365026

Changes: https://git.openjdk.org/jdk/pull/26673/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26673&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365026
  Stats: 8 lines in 3 files changed: 3 ins; 4 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/26673.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26673/head:pull/26673

PR: https://git.openjdk.org/jdk/pull/26673