t1300: fix the new --show-origin tests on Windows

t1300: fix the new --show-origin tests on Windows

On Windows, we have that funny situation where the test script can refer
to POSIX paths because it runs in a shell that uses a POSIX emulation
layer ("MSYS2 runtime"). Yet, git.exe does *not* understand POSIX paths
at all but only pure Windows paths.

So let's just convert the POSIX paths to Windows paths before passing
them on to Git, using `pwd` (which is already modified on Windows to
output Windows paths).

While fixing the new tests on Windows, we also have to exclude the tests
that want to write a file with a name that is illegal on Windows
(unfortunately, there is more than one test trying to make use of that
file).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>