Use jgit to get git describe for the kernel version

Build-time wise this equals having ant call out to CLI git, but I think using JGit will prove more stable. (that call gets stuck a lot on my Ubuntu boxes.) Also, we'll get -dirty appended to the revision string when building from a dirty repo. (couldn't do that previously, as we didn't know what git version would be in use)

Here's the generated class from the PR build 118:

```java
package org.neo4j.kernel.impl;

import org.neo4j.kernel.Version;
import org.neo4j.helpers.Service;

@Service.Implementation(Version.class)
public class ComponentVersion extends Version
{
    public ComponentVersion()
    {
        super("neo4j-kernel", "1.9-SNAPSHOT");
    }

    @Override
    public String getRevision()
    {
        return "1.9.M02-237-g072d5cd";
    }
}
```