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ARROW-2516: [CI] Filter changes in AppVeyor builds

In AppVeyor PR builds, jobs can be exited early if they test something that the PR doesn't affect.

Author: Antoine Pitrou <antoine@python.org>

Closes #1989 from pitrou/ARROW-2516-appveyor-filter-changes and squashes the following commits:

635cd74 <Antoine Pitrou> ARROW-2516:  Filter changes in AppVeyor builds