Improve JGIT test prereq (jgit on Fedora >= 30 is broken)
    
    Running jgit on Fedora >= 30 results in an immediate failure¹:
    
      $ jgit --version
      /usr/bin/build-classpath: Could not find xz-java Java extension for this JVM
      /usr/bin/build-classpath: error: Some specified jars were not found
      Error: Could not find or load main class org.springframework.boot.loader.JarLauncher
    
    Skip the jgit tests if 'jgit --version' fails.  This way we'll begin
    running them again once the issue is resolved -- without having to make
    any further changes to the git package.
    
    Also exclude jgit on i386 arch, as upstream eclipse has dropped support.
    We could adjust the conditional to only exclude on Fedora >= 30 and
    i386, but the added complexity is not worth the effort.
    
    ¹ jgit bug report: https://bugzilla.redhat.com/1709624