RFR: 8295267: Improve G1 merge to scan ratio calculation

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that fixes the merge-to-scan ratio calculation ("which fraction of the remembered sets creates unique cards") for predictions to make it more accurate and understandable?

Previously the denominator has been the total number of scanned cards; this seems to be a good approximation (trying to make a case for it, it's wrong) if the number of log buffers is low at GC start, but tends to get broken otherwise (i.e. with [JDK-8137022](https://bugs.openjdk.org/browse/JDK-8137022)).

This change fixes this to use the number of cards that were merged in.

This change depends on PR#10656. Please review that one first if not done otherwise.

Testing: tier1, checking that no tests use the "Dirty Cards" string now changed.

Thanks,
  Thomas

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Depends on: https://git.openjdk.org/jdk/pull/10656

Commit messages:
 - Improve naming, comments
 - Remove debug code
 - Fix test
 - initial version

Changes: https://git.openjdk.org/jdk/pull/10694/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10694&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295267
  Stats: 34 lines in 6 files changed: 4 ins; 11 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/10694.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10694/head:pull/10694

PR: https://git.openjdk.org/jdk/pull/10694