[MERGE][bug #6700] diff on branches without working trees

[Aaron Bentley]

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Ian Clatworthy wrote:
>> ^^^ technically, you could try WorkingTree.revision_tree before falling
>> back to Repository.revision_tree.  But that would only accelerate diff
>> -r -1, which is rarely used.
> 
> Interesting. How would you change that bit of code to do that? LBYL? I
> don't want to make the code more complex that it need be or risk slowing
> down common cases, but I'm happy to tweak it if you think it's worth it.

I don't think it's worth it.  But it would be:

try:
    return tree.revision_tree(revision_id)
except errors.NoSuchRevision:
    return rev_branch.repository.revision_tree(revision_id)

The expense of generating a RevisionTree dwarfs the expense of catching
the exception.

Aaron
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