Add new git issue with falling back to cwd if a command is not in $PATH

A possible attack scenario consist of a user operating on a specially
crafted repository containing such a mailicious script and the user not
having an installation of git with a particular git command installed
but operate on that git command later on while beeing in the repository.

For instance say git-email might not be installed, the repository
containing git-send-email and the user using git send-email later on.