Mark package org.eclipse.jgit.gitrepo.internal as x-internal

It's an internal package which isn't part of the API. Mark it x-internal
to silence @since tag warnings which are only raised for new API.

Bug: 440757
Change-Id: Id05deaca43f135cd1bfe83cf1f29787cbbdbecac
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/META-INF/MANIFEST.MF

  org.eclipse.jgit.gitrepo;version="3.5.0";
   uses:="org.eclipse.jgit.api,
    org.eclipse.jgit.lib",
- org.eclipse.jgit.gitrepo.internal;version="3.5.0",
+ org.eclipse.jgit.gitrepo.internal;version="3.5.0";x-internal:=true,
  org.eclipse.jgit.ignore;version="3.5.0",
  org.eclipse.jgit.internal;version="3.5.0";x-friends:="org.eclipse.jgit.test,org.eclipse.jgit.http.test",
  org.eclipse.jgit.internal.storage.dfs;version="3.5.0";x-friends:="org.eclipse.jgit.test",