add `/etc/gitconfig` for better git security

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• GHSA-8prw-h3cq-mghm
• https://forums.whonix.org/t/git-users-enable-fsck-by-default-for-better-security/2066

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Merged already but leaving this open for discussion, review, suggestions, etc.

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Why is `fsckobjects = true` repeated twice in transfer, fetch, receive?\ Shouldn't it only be once for each?

Thanks. Fixed in git.