t1401: make invocation of tar(1) work with Win32-provided one

t1401: make invocation of tar(1) work with Win32-provided one

Windows nowadays provides a tar(1) binary in "C:\Windows\system32". This
version of tar(1) doesn't seem to handle the case where directory paths
end with a trailing forward slash. And as we do that in t1401 the result
is that the test fails.

Drop the trailing slash. Other tests that use tar(1) work alright, this
is the only instance where it has been failing.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Taylor Blau <me@ttaylorr.com>