Jgit doesn't shutdown thread executors

[spyrkob]

When using prospero in maven exec plugin, the execution reports lingering threads:

[WARNING] thread Thread[JGit-FileStoreAttributeReader-1,5,org.wildfly.prospero.cli.CliMain] was interrupted but is still alive after waiting at least 15000msecs
[WARNING] thread Thread[JGit-FileStoreAttributeReader-1,5,org.wildfly.prospero.cli.CliMain] will linger despite being asked to die via interruption
[WARNING] NOTE: 1 thread(s) did not finish despite being asked to via interruption. This is not a problem with exec:java, it is a problem with the running code. Although not serious, it should be remedied.
[WARNING] Couldn't destroy threadgroup org.codehaus.mojo.exec.ExecJavaMojo$IsolatedThreadGroup[name=org.wildfly.prospero.cli.CliMain,maxpri=10]
java.lang.IllegalThreadStateException
    at java.lang.ThreadGroup.destroy (ThreadGroup.java:776)
    at org.codehaus.mojo.exec.ExecJavaMojo.execute (ExecJavaMojo.java:331)
    at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:126)
    at org.apache.maven.lifecycle.internal.MojoExecutor.doExecute2 (MojoExecutor.java:328)
    at org.apache.maven.lifecycle.internal.MojoExecutor.doExecute (MojoExecutor.java:316)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:212)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:174)

This seems to be caused by jgit executors not being shut down. Part of fix is calling Git.shutdown(), but org.eclipse.jgit.util.FS executors are still not shutting down.

Below is an example pom.xml:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.test.prospero</groupId>
    <artifactId>prospero-test</artifactId>
    <version>1.0.0.Final-SNAPSHOT</version>
    <packaging>jar</packaging>

    <dependencies>
        <dependency>
            <groupId>org.wildfly.prospero</groupId>
            <artifactId>prospero-cli</artifactId>
            <version>1.2.2.Final-SNAPSHOT</version>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>exec-maven-plugin</artifactId>
                <executions>
                    <execution>
                        <id>multi-channel-provisioning-ee</id>
                        <phase>compile</phase>
                        <goals>
                            <goal>java</goal>
                        </goals>
                        <configuration>
                            <mainClass>org.wildfly.prospero.cli.CliMain</mainClass>
                            <commandlineArgs>install --dir=${project.build.directory}/wildfly-multi-channel --profile=wildfly --manifest=${project.basedir}/wildfly-29.0.0.Final-manifest.yaml</commandlineArgs>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
</project>