[MERGE] make it possible to pass NULL_REVISION as the branch_tip parameter to merge_sort

[Aaron Bentley]

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John Arbash Meinel wrote:
> Aaron Bentley wrote:
> | Well, Graph already provides iter_topo_order.
> 
> Sure, though iter_topo_order is topo_sort order and not merge_sort
> order. Plus
> it doesn't return revision numbers.

But merge_sort is built on top of topo_sort, so it should be a simple port.

> | Weren't we just recently talking about moving merge_sort onto Graph?
> I don't remember a discussion on that, but yeah, that would be reasonable.

Hmm.  I can't find the thread right now, but it was about generating
dotted revnos for gannotate.  Or maybe I just *meant* to suggest it.

> get_revision_graph is a great candidate for being deprecated.

So everywhere we need a whole-history graph we just do
dict(Graph.iter_ancestry) ?

Aaron
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