mingw: make test_path_is_hidden more robust

This function uses Windows' system tool `attrib` to determine the state
of the hidden flag of a file or directory.

We should not actually expect the first `attrib.exe` in the PATH to
be the one we are looking for. Or that it is in the PATH, for that
matter.

Let's use the full path to the tool instead.

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

t/test-lib-functions.sh

@@ -1547,6 +1547,6 @@ test_bitmap_traversal () {
 # Tests for the hidden file attribute on windows
 is_hidden () {
 	# Use the output of `attrib`, ignore the absolute path
-	case "$(attrib "$1")" in *H*?:*) return 0;; esac
+	case "$("$SYSTEMROOT"/system32/attrib "$1")" in *H*?:*) return 0;; esac
 	return 1
 }