fix TARGET path handling

[tomslobodnik]

mkstage4 stopped working for me after commit d1372b0 by

1. being dependent on current working directory, and
2. not matching default exclude patterns because of double slash prefix when used with -s option

Following change works with both -s and -t options (gentoo, tar 1.29, bash 4.4.12). When -t is used, trailing slash is added if not present to make solution universal.

[TheChymera]

Looking good!

As mentioned in another PR having some tests for this would prevent commits unintentionally breaking some popular use cases. Would you like to help out with this? Ideally we would be looking for some unit tests which can use gentoo images.

[tomslobodnik]

I agree having tests is important to prevent simple changes breaking down the script. I would like to help, but can't promise anything due to my very limited free time and lack of experience with CI and unit testing. If you have more specific idea about the tests, it would help me.