t7300: fix clean up on Windows

On Windows, you cannot remove files that are in use, not even with
'rm -rf'.  So we need to run 'exec <foo/bar' inside a subshell lest
removing the whole test repository fail.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>

t/t7300-clean.sh

@@ -373,9 +373,9 @@ test_expect_success 'removal failure' '
 
 	mkdir foo &&
 	touch foo/bar &&
-	exec <foo/bar &&
-	chmod 0 foo &&
-	test_must_fail git clean -f -d
+	(exec <foo/bar &&
+	 chmod 0 foo &&
+	 test_must_fail git clean -f -d)
 
 '
 chmod 755 foo