[MERGE] pull and merge commands can use merge directives

[Aaron Bentley]

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Martin Pool wrote:
> bb gives me an error; i was trying to say:
> 
> +0:
> 
> Well, I think being able to feed them into merge and pull would be very
> nice.
> 
> I am a bit concerned by read_bundle_or_directive- couldn't we just
> somehow return a single type of object that does either thing?

Well, we can't return BundleReader, because the merge directive may not
contain a bundle.  We can't return a MergeDirective, because the
testament_sha1 for a v0.8 bundle is not right.  Also, in the current
implementation, it would mean throwing out the reader.

> The help for merge-directive, pull and merge should probably say that
> you can do this.
> 
> I don't understand what it means to give -r and a directive file, and
> that should be in the help too.

Ah.  Well, it should mean the same thing as in means in the context of
the command, given a branch argument.  The merge directive should stand
in for the branch that the merge directive was generated from.

Aaron
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