Integrated: 7902794: Provide a `release` file for JT Harness builds

Wed Nov 11 19:37:01 UTC 2020

generating 'release' file for any call of 'jar' target (or any target depending on 'jar') with approximately the following content:

JTHARNESS_VERSION=6.0 b15
BUILD_DATE=Nov 11, 2020
SOURCE=jtharness:04050d3f6754491a67c71a7a98a34a7a00d4eb62

The last line won't be included if 'git' is not found on the system path.
Still assuming that jtharness repository is under Git VCS

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Commit messages:
 - 7902794: Provide a `release` file for JT Harness builds

Changes: https://git.openjdk.java.net/jtharness/pull/8/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtharness&pr=8&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7902794
  Stats: 19 lines in 1 file changed: 17 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jtharness/pull/8.diff
  Fetch: git fetch https://git.openjdk.java.net/jtharness pull/8/head:pull/8

PR: https://git.openjdk.java.net/jtharness/pull/8