windows: fix Repository>Explore Working Copy

[dscho]

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Since Git v2.39.1, we are a bit more stringent in searching the PATH. In particular, we specifically require the .exe suffix.

However, the Repository>Explore Working Copy command asks for explorer.exe to be found on the PATH, which already has that suffix.

Let's unstartle the PATH-finding logic about this scenario.

This fixes https://github.com/git-for-windows/git/issues/4356

[rimrul]

This also fixes #94.