Stop modifying and reusing the global JavaHg RepositoryConfiguration singleton

I can't point to a specific error, but modifying a singleton and reusing it is
a good way to get unexpected state.  The extension adding won't collect
additional state (it's add-if-not-present), but the underlying
`java.util.HashMap` isn't threadsafe.  Any differences in the environment map
when this is called would alter that state of anything else that still held a
configuration (and it also uses `HashMap`), and the pending changeset, encoding,
and HgBin settings would be overwritten for everybody outright.

There's only a default constructor for this class, so nothing to pass along in
the constructor.  I don't *think* this was the cause of the random auth failures
mentioned in the previous commit, but it's easy to avoid these potential
problems.

scm-plugins/scm-hg-plugin/src/main/java/sonia/scm/repository/HgRepositoryFactory.java

@@ -59,7 +59,7 @@ public class HgRepositoryFactory {
     HgConfig config = configResolver.resolve(repository);
     File directory = config.getDirectory();
 
-    RepositoryConfiguration repoConfiguration = RepositoryConfiguration.DEFAULT;
+    RepositoryConfiguration repoConfiguration = new RepositoryConfiguration();
     repoConfiguration.setHgrcPath(null);
     repoConfiguration.getEnvironment().putAll(environment);
     repoConfiguration.addExtension(HgFileviewExtension.class);