Fail if attempting to execute git commands in a directory that is not…

… a git-repo. (stefanzweifel#326)

* Fail (and log message) if attempting to execute git commands in a directory that is not a git-repo.

* Add Test

* Code Formatting

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Co-authored-by: Constantin Comendant <constantin@uberagent.com>
Co-authored-by: Stefan Zweifel <stefan@stefanzweifel.dev>