t: consolidate the is_hidden functions

The `is_hidden` function can be used (only on Windows) to determine
whether a directory or file have their `hidden` flag set.

This function is duplicated between two test scripts. It is better to
move it into `test-lib-functions.sh` so that it is reused.

This patch is best viewed with `--color-moved`.

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

t/test-lib-functions.sh

@@ -1543,3 +1543,10 @@ test_bitmap_traversal () {
 	test_cmp "$1.normalized" "$2.normalized" &&
 	rm -f "$1.normalized" "$2.normalized"
 }
+
+# Tests for the hidden file attribute on windows
+is_hidden () {
+	# Use the output of `attrib`, ignore the absolute path
+	case "$(attrib "$1")" in *H*?:*) return 0;; esac
+	return 1
+}