Fix exception on import in MacOS · gitpython-developers/GitPython@ce5dfe7

Fix exception on import in MacOS

This is related to my fix in #658. Apparently, MacOS adds a git executable that is just a stub which displays an error. This gets past the try/except I added in #658, and allows all of the GitPython components to be imported, but since the executable is not *actually* git, it results in an exception when ``refresh()`` attemepts to run a ``git version``.

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