The rebase dialog was updated recently with an autocomplete for change
numbers. Refs and commit hashes never appear in the autocomplete, thus
that change disabled the ability to rebase onto those types of entries.
This change adds an additional parameter to allow users to input values
other than the suggested change numbers, e.g. a ref.
Bug: Issue 8412
Change-Id: I90173641d0f0127abdb94edb994249851e4a2ca8