mimetype is super slow for big binary filenodes

[lukaszb]

Ok like in the title, i found that the node.is_binary function really messes things up.

{{{

!python

@LazyProperty
def is_binary(self):
    """
    Returns True if file has binary content.
    """          
    return bool(self.content and '\0' in self.content)

}}}

It looks like this is a CPU killer, for content files with let's say 70MB binary file.

I think it's need to be rewrite the to a lazy read in chunk generator for the content. That should speed up things a lot.

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Reference: https://bitbucket.org/marcinkuzminski/vcs/issues/37/