There is a discrepancy of behavior of `github.com/urfave/cli` between
using go1.12 and go1.15, when the dependency is not present as vendored
source. Now this builds fine with go1.12
There are users of tar-split as a package. It is the hope that by adding
this vendored source it does not impact them depending on tar-split
itself.
Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>
