If we split a filename into `Name` and `Prefix`, `Prefix` is at most
145 bytes. We had a bug that didn't split a path correctly. This bug
was pointed out by Rafael in the post commit review.
This patch adds a unit test for TarWriter to verify the fix.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291494 91177308-0d34-0410-b5e6-96231b3b80d8