RFR: 8347334: JimageDiffGenerator code clean-ups

Thu Jan 9 17:37:50 UTC 2025

During code review of [JDK-8346239](https://bugs.openjdk.org/browse/JDK-8346239) a few comments were made after the PR integrated. This follow-up patch cleans this up and adds a unit test for the `JimageDiffGenerator` class.

Testing:
- [ ] GHA
- [x] tools/jlink tests (including the new one).

Thoughts?

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Commit messages:
 - 8347334: JimageDiffGenerator code clean-ups

Changes: https://git.openjdk.org/jdk/pull/23014/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23014&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347334
  Stats: 319 lines in 2 files changed: 291 ins; 12 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/23014.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23014/head:pull/23014

PR: https://git.openjdk.org/jdk/pull/23014