add support for `git worktree add`

as described here https://stackoverflow.com/questions/6270193/how-can-i-have-multiple-working-directories-with-git/30185564#30185564 git can create separate directories off the same .git folder.

when trying to add one of those directories to smartgit,  you get the message:
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Add or Create Repository
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Should 'Revit' be initialized as a new Git repository?
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The specified directory does not appear to be a valid Git repository.
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[[&Initialize]] [Cancel]
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likely because it does not recognize the .git FILE.