restore: use case insensitive file name comparison on windows

internal/restorer/restorer_windows.go

@@ -0,0 +1,13 @@
+//go:build windows
+// +build windows
+
+package restorer
+
+import "strings"
+
+// toComparableFilename returns a filename suitable for equality checks. On Windows, it returns the
+// uppercase version of the string. On all other systems, it returns the unmodified filename.
+func toComparableFilename(path string) string {
+	// apparently NTFS internally uppercases filenames for comparision
+	return strings.ToUpper(path)
+}