Getting back to the repository removal handling from Sunday, I made the
assistant detect when a repository that has been marked as unwanted becomes
empty, and finish the removal process.

I was able to add this to the expensive transfer scan without making it any
more expensive than it already was, since that scan already looks at the
location of all keys. Although when a remote is detected as empty, it then
does one more check, equivilant to `git annex unused`, to find any
remaining objects on the remote, and force them off.

I think this should work pretty well, but it needs some testing and
probably some UI work.

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Andy spotted a bug in the preferred content expressions I was using to
handle untrusted remotes. So he saved me several hours dealing with an ugly
bug at some point down the line. I had misread my own preferred content
expression documentation, and `copies=semitrusted:1` was not doing what I
thought it was. Added a new syntax that does what I need,
`copies=semitrusted+:1`

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The 64 bit linux standalone builds are back. Apparently the 32 bit builds
have stopped working on recent Fedora, for reasons that are unclear. I set
up an autobuilder to produce the 64 bit builds.
