Revision API: Add method to query if revision must be rebased

With canRebase() method plugins can figure out if a specific revision
can actually be rebased, if it's not based on the latest revision of
the dependent change.

  boolean canRebase = gApi.changes()
      .id(change)
      .revision(revision)
      .canRebase();

Change-Id: Ib64a6ed5a62f83639143088624ad72d93edae3cf
3 files changed