t: restrict `is_hidden` to be called only on Windows

The function won't work anywhere else, so let's mark it as an explicit
bug if it is called on a non-Windows platform.

Let's also rename the function to avoid cluttering the global namespace
with an overly-generic function name.

While at it, we also fix the code comment above that function: the
lower-case `windows` refers to something different than `Windows`.

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

t/t0001-init.sh

@@ -399,7 +399,7 @@ test_expect_success MINGW '.git hidden' '
 		mkdir newdir &&
 		cd newdir &&
 		git init &&
-		is_hidden .git
+		test_path_is_hidden .git
 	) &&
 	check_config newdir/.git false unset
 '

t/t5611-clone-config.sh

@@ -96,13 +96,13 @@ test_expect_success MINGW 'clone -c core.hideDotFiles' '
 	test_commit attributes .gitattributes "" &&
 	rm -rf child &&
 	git clone -c core.hideDotFiles=false . child &&
-	! is_hidden child/.gitattributes &&
+	! test_path_is_hidden child/.gitattributes &&
 	rm -rf child &&
 	git clone -c core.hideDotFiles=dotGitOnly . child &&
-	! is_hidden child/.gitattributes &&
+	! test_path_is_hidden child/.gitattributes &&
 	rm -rf child &&
 	git clone -c core.hideDotFiles=true . child &&
-	is_hidden child/.gitattributes
+	test_path_is_hidden child/.gitattributes
 '
 
 test_done

t/test-lib-functions.sh

@@ -1544,8 +1544,11 @@ test_bitmap_traversal () {
 	rm -f "$1.normalized" "$2.normalized"
 }
 
-# Tests for the hidden file attribute on windows
-is_hidden () {
+# Tests for the hidden file attribute on Windows
+test_path_is_hidden () {
+	test_have_prereq MINGW ||
+	BUG "test_path_is_hidden can only be used on Windows"
+
 	# Use the output of `attrib`, ignore the absolute path
 	case "$("$SYSTEMROOT"/system32/attrib "$1")" in *H*?:*) return 0;; esac
 	return 1