[jdk11u-dev] RFR: 8278851: Correct signer logic for jars signed with multiple digestalgs

[Goetz Lindenmaier]

I backport this for parity with 11.0.16-oracle.
This is based on the jdk17u-dev change as it is closer to 11 than the original.

I had to resolve because of differences in Hex formatting in context lines.

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Commit messages:
 - Backport cbe497394786ff76a09f9743040e3ba96ee8298f

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/928/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=928&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278851
  Stats: 272 lines in 4 files changed: 221 ins; 19 del; 32 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/928.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/928/head:pull/928

PR: https://git.openjdk.java.net/jdk11u-dev/pull/928