RFR: CODETOOLS-7903220: Improve setting of the generated "release" file

[Jonathan Gibbons]

Please review a small change to permit  more flexible, optional setting of the `.src-rev` file used to generate info in the `release` file for an image.

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Commit messages:
 - CODETOOLS-7903220: Improve setting of the generated "release" file

Changes: https://git.openjdk.org/jtreg/pull/89/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=89&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903220
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jtreg/pull/89.diff
  Fetch: git fetch https://git.openjdk.org/jtreg pull/89/head:pull/89

PR: https://git.openjdk.org/jtreg/pull/89