[MERGE] don't invoke os.rename on children with no contents

[Aaron Bentley]

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John Arbash Meinel wrote:
> John Arbash Meinel has voted +1.
> Status is now: Approved
> Comment:
> when are things canceled?

It looks like the only use in the core code is Merge3Merger.fix_root.
When a merge tries to produce a tree with two roots, we move the new
root's children into the current root, then cancel the new root.

It is also used in the three-way-merge tests in
test_merge_core.MergeBuilder.

I thought we could also do it during conflict resolution, but I guess I
was wrong.

Aaron
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