Incrementalify plugins

[basil]

It is considered a best practice for Jenkins plugins to implement incrementals. Among other benefits, this allows for easy testing of PR builds and is a necessary prerequisite for setting up automated release. The steps are, roughly:

• If missing, create .mvn/extensions.xml with the latest version of git-changelist-maven-extension.
• If missing, add .mvn/maven.config with -Pconsume-incrementals and -Pmight-produce-incrementals.
  • Note that -Dchangelist.format is also needed for automated release, but not necessary for the traditional release process.
• If a version like <version>1.20-SNAPSHOT</version> is present, change this to <version>${revision}${changelist}</version> and add <revision>1.20</revision> and <changelist>-SNAPSHOT</changelist> to the <properties> section.
  • Note that this could be set to just <version>${changelist}</version> or even <version>${revision}.${changelist}</version> for automated release as described here.
• If missing, add <gitHubRepo>jenkinsci/${project.artifactId}-plugin</gitHubRepo> to the <properties> section.
  • Note that some plugins (not many, though) have an artifact ID that does not match their GitHub repository, so in that case one cannot use ${project.artifactId}.
• Ensure the <scm> section reads like this:

<scm>
  <connection>scm:git:https://github.com/${gitHubRepo}.git</connection>
  <developerConnection>scm:git:git@github.com:${gitHubRepo}.git</developerConnection>
  <tag>${scmTag}</tag>
  <url>https://github.com/${gitHubRepo}</url>
</scm>

Incrementalifying a Java 9+ plugin for the first time will likely also expose #14.