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

[Iris Clark]

On Wed, 29 Jun 2022 17:14:42 GMT, Jonathan Gibbons <jjg at openjdk.org> wrote:

> 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.

Marked as reviewed by iris (Reviewer).

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