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>

t/t1508-at-combinations.sh

@@ -35,7 +35,10 @@ test_expect_success 'setup' '
 	git checkout -b upstream-branch &&
 	test_commit upstream-one &&
 	test_commit upstream-two &&
-	git checkout -b @/at-test &&
+	if test_have_prereq !MINGW
+	then
+		git checkout -b @/at-test
+	fi &&
 	git checkout -b @@/at-test &&
 	git checkout -b @at-test &&
 	git checkout -b old-branch &&
@@ -64,6 +67,7 @@ check "@{-1}@{u}@{1}" commit master-one
 check "@" commit new-two
 check "@@{u}" ref refs/heads/upstream-branch
 check "@@/at-test" ref refs/heads/@@/at-test
+test_have_prereq MINGW ||
 check "@/at-test" ref refs/heads/@/at-test
 check "@at-test" ref refs/heads/@at-test
 nonsense "@{u}@{-1}"