fix: list objects trim common prefixes that match marker prefix

This checks to see if the common prefix is before the marker and
thus would have been returned in earlier list objects request.

The error case was aws cli listing multiple entries for the same
common prefix when the listing required multiple pagination
requests.

Fixes #778

backend/walk.go

@@ -191,12 +191,19 @@ func Walk(ctx context.Context, fileSystem fs.FS, prefix, delimiter, marker strin
 
 		// Common prefixes are a set, so should not have duplicates.
 		// These are abstractly a "directory", so need to include the
-		// delimiter at the end.
+		// delimiter at the end when we add to the map.
+		cprefNoDelim := prefix + before
 		cpref := prefix + before + delimiter
 		if cpref == marker {
 			pastMarker = true
 			return nil
 		}
+
+		if marker != "" && strings.HasPrefix(marker, cprefNoDelim) {
+			// skip common prefixes that are before the marker
+			return nil
+		}
+
 		cpmap[cpref] = struct{}{}
 		if (len(objects) + len(cpmap)) == int(max) {
 			newMarker = cpref