Only gc.collect() under windows

Under Windows, tempfile objects are holding references to open files
until the garbage collector closes them and frees them.  Explicit
calls to gc.collect() were added to the finalizer for the Repo class
to force them to be closed synchronously.  However, this is expensive,
especially in large, long-running programs.  As a temporary measure
to alleviate the performance regression on other platforms, only
perform these calls when running under Windows.

Fixes #553

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