1. fixes comparisons with true for flag variables
2. renames TMPDIR to NGCI_TMPDIR so it doesn't interfere with the
standardized TMPDIR var (and at some point accidentally deleting /tmp)
3. add small help message when invalid arg/param is passed
4. run `git add .` on copied git repo in /tmp so staged files stay
staged there aswell
the goal was to remove "variables" when it's empty, but this resulted in
the original value being used if all of the variables contained store
paths. This is not supposed to happen and thus fixed with this.
