git worktree with jgitver

## Issue

**1.2.1**: _replace with the version of the jgtiver-maven-plugin you have the issue with_

**usage context**:

- [x] maven command line: 3.5.2
- [ ] eclipse: _replace with version_
- [ ] netbeans: _replace with version_
- [ ] Intellij IDEA: _replace with version_

**Problem description**:

- I use `git worktree` since a few weeks, and when I try to build with maven, I have this error :
```

[WARNING] cannot autoclose GitVersionCalculator object for project: /home/dev/projet/fix
java.lang.IllegalStateException: failure calculating version
at fr.brouillard.oss.jgitver.GitVersionCalculator.buildVersion (GitVersionCalculator.java:312)
at fr.brouillard.oss.jgitver.GitVersionCalculator.getVersionObject (GitVersionCalculator.java:144)
at fr.brouillard.oss.jgitver.GitVersionCalculator.getVersion (GitVersionCalculator.java:203)
at fr.brouillard.oss.jgitver.GitVersionCalculator.meta (GitVersionCalculator.java:512)
at fr.brouillard.oss.jgitver.JGitverExtension.afterSessionStart (JGitverExtension.java:93)
at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:210)
at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:194)
at org.apache.maven.DefaultMaven.execute (DefaultMaven.java:107)
at org.apache.maven.cli.MavenCli.execute (MavenCli.java:955)
at org.apache.maven.cli.MavenCli.doMain (MavenCli.java:290)
at org.apache.maven.cli.MavenCli.main (MavenCli.java:194)
at sun.reflect.NativeMethodAccessorImpl.invoke0 (Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke (NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke (DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke (Method.java:498)
at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced (Launcher.java:289)
at org.codehaus.plexus.classworlds.launcher.Launcher.launch (Launcher.java:229)
at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode (Launcher.java:415)
at org.codehaus.plexus.classworlds.launcher.Launcher.main (Launcher.java:356)
Caused by: org.eclipse.jgit.errors.NoWorkTreeException: Bare Repository has neither a working tree, nor an index
at org.eclipse.jgit.lib.Repository.getWorkTree (Repository.java:1356)
at org.eclipse.jgit.treewalk.FileTreeIterator. (FileTreeIterator.java:115)
at org.eclipse.jgit.treewalk.FileTreeIterator. (FileTreeIterator.java:97)
at org.eclipse.jgit.api.StatusCommand.call (StatusCommand.java:139)
at fr.brouillard.oss.jgitver.impl.GitUtils.isDirty (GitUtils.java:51)
at fr.brouillard.oss.jgitver.GitVersionCalculator.buildVersion (GitVersionCalculator.java:209)
at fr.brouillard.oss.jgitver.GitVersionCalculator.getVersionObject (GitVersionCalculator.java:144)
at fr.brouillard.oss.jgitver.GitVersionCalculator.getVersion (GitVersionCalculator.java:203)
at fr.brouillard.oss.jgitver.GitVersionCalculator.meta (GitVersionCalculator.java:512)
at fr.brouillard.oss.jgitver.JGitverExtension.afterSessionStart (JGitverExtension.java:93)
at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:210)
at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:194)
at org.apache.maven.DefaultMaven.execute (DefaultMaven.java:107)
at org.apache.maven.cli.MavenCli.execute (MavenCli.java:955)
at org.apache.maven.cli.MavenCli.doMain (MavenCli.java:290)
at org.apache.maven.cli.MavenCli.main (MavenCli.java:194)
at sun.reflect.NativeMethodAccessorImpl.invoke0 (Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke (NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke (DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke (Method.java:498)
at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced (Launcher.java:289)
at org.codehaus.plexus.classworlds.launcher.Launcher.launch (Launcher.java:229)
at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode (Launcher.java:415)
at org.codehaus.plexus.classworlds.launcher.Launcher.main (Launcher.java:356)
```

But I think it´s more about jGit [jgit issue](https://bugs.eclipse.org/bugs/show_bug.cgi?id=477475)