mingw: skip test in t1508 that fails due to path conversion

mingw: skip test in t1508 that fails due to path conversion

In Git for Windows, the MSYS2 POSIX emulation layer used by the Bash
converts command-line arguments that looks like they refer to a POSIX
path containing a file list (i.e. @<absolute-path>) into a Windows path
equivalent when calling non-MSYS2 executables, such as git.exe.

Let's just skip the test that uses the parameter `@/at-test` that
confuses the MSYS2 runtime.

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