Support pluggable Git integration

[tisonkun]

license-maven-plugin has a pluggable Git integration license-maven-plugin-git.

We can have a similar plugin to populate Git-related information like first commit time, last commit time and excludes from .gitignore rules, etc.

It will require a plugin strategy and perhaps dynamically loaded. But in the native image and command line fat jar, we may bundle them. I'm unsure now.

For Git interoperation library, jgit is a good choice: https://www.eclipse.org/jgit/

[spencercjh]

More details:

license-maven-plugin-git is a jar which will be added to the dependencies of the plugin.

<plugin>
    <groupId>com.mycila</groupId>
    <artifactId>license-maven-plugin</artifactId>
    <version>${license-maven-plugin.version}</version>
    <configuration>
      <header>my-header-folder/my-header-template.txt</header>
     ...
    </configuration>
    <dependencies>
      <dependency>
        <groupId>com.mycila</groupId>
        <artifactId>license-maven-plugin-fs</artifactId>
        <!-- make sure you use the same version as license-maven-plugin -->
        <version>${license-maven-plugin.version}</version>
      </dependency>
    </dependencies>
    <executions>
      ...
    </executions>
</plugin>

It implements the interface PropertiesProvider and will be loaded here

for (final PropertiesProvider provider : ServiceLoader.load(PropertiesProvider.class,
    Thread.currentThread().getContextClassLoader())) {
  provider.init(this, globalProperties);
  propertiesProviders.add(provider);
}

[tisonkun]

Yes. Another important improvement is filtering out files that ignored by .gitignore file.

But for this purpose we may firstly factor out the Selection logics to support file filter SPIs.