Add again some utilities to support incrementals via Jenkins infra pipeline-library

[alextu]

This is another follow up of https://github.com/jenkinsci/gradle-jpi-plugin/issues/215 that is not sufficient to publish incrementals when running on ci.jenkins.io (hopefully the last one ?).

• It requires first to have <tag>...</tag> set in the published pom with the full git hash. This PR changes the generateGitVersionTask to also set the full Git hash in the version.txt in addition to the formatted one. It also adds ascmTag property to the JpiExtension that can be set via a Gradle property. Eventually the https://github.com/jenkins-infra/pipeline-library/blob/master/vars/buildPluginWithGradle.groovy will do something like -PscmTag=${versionFile.readLines()[1]}

• Optionally users should be able to add a version prefix to the generate version, like a "major.minor" for example, ending up with for example 1.0-rc1.abcdef or a real world example: https://ci.jenkins.io/job/Plugins/job/git-plugin/job/master/ This PR adds an optional versionPrefix to the GitVersionExtension

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• [x] Please describe what you did

• [x] Link to relevant issues in GitHub or Jira

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• [x] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[sghill]

Thanks so much for following up on this @alextu. Getting these contributions is wonderful.

One question on this line - should we support windows too by using String.format("%n")?

I have released this change as 0.49.0-rc.2 today.

[alextu]

Thanks so much for following up on this @alextu. Getting these contributions is wonderful.

One question on this line - should we support windows too by using String.format("%n")?

I have released this change as 0.49.0-rc.2 today.

Thanks! Good catch, I will submit a PR to fix this