prev: prompt user to choose parent in case of multiple parents This patch adds functionality to `hg prev` to prompt user to choose one parent to update when multiple parents exist. Surprisingly there were no tests for this case before this patch, I added one with this patch.

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−# name space package to host third party extensions
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−from __future__ import absolute_import
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−import pkgutil
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−__path__ = pkgutil.extend_path(__path__, __name__)