RFR: 8259559: COMPARE_BUILD can't compare patch files

[Magnus Ihse Bursie]

COMPARE_BUILD will fail to run the compare stage, since re-applying an applied patch does not work.

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Commit messages:
 - 8259559: COMPARE_BUILD can't compare patch files

Changes: https://git.openjdk.java.net/jdk/pull/2025/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2025&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259559
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2025.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2025/head:pull/2025

PR: https://git.openjdk.java.net/jdk/pull/2025