RFR: 7903760: Revert "7903622: Remove support for `jtdiff`"

[Christian Stein]

This reverts commit a8698b72b3695778c946752fe22c1f6fadfbe936.

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Commit messages:
 - Revert "7903622: Remove support for `jtdiff`"

Changes: https://git.openjdk.org/jtreg/pull/209/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=209&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903760
  Stats: 3322 lines in 22 files changed: 3321 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jtreg/pull/209.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/209/head:pull/209

PR: https://git.openjdk.org/jtreg/pull/209