Fix incorrect merging of slash-suffixed objects (minio#19699)

If two objects share everything but one object has a slash prefix, those would be merged in listings, 
with secondary properties used for a tiebreak.

Example: An object with the key `prefix/obj` would be merged with an object named `prefix/obj/`. 
While this violates the [no object can be a prefix of another](https://min.io/docs/minio/linux/operations/concepts/thresholds.html#conflicting-objects), let's resolve these.

If we have an object with 'name' and a directory named 'name/' discard the directory only - but allow objects 
of 'name' and 'name/' (xldir) to be uniquely returned.

Regression from minio#15772

cmd/metacache-entries.go

@@ -736,16 +736,35 @@ func mergeEntryChannels(ctx context.Context, in []chan metaCacheEntry, out chan<
 				bestIdx = otherIdx
 				continue
 			}
-			// We should make sure to avoid objects and directories
-			// of this fashion such as
-			//  - foo-1
-			//  - foo-1/
-			// we should avoid this situation by making sure that
-			// we compare the `foo-1/` after path.Clean() to
-			// de-dup the entries.
 			if path.Clean(best.name) == path.Clean(other.name) {
-				toMerge = append(toMerge, otherIdx)
-				continue
+				// We may be in a situation where we have a directory and an object with the same name.
+				// In that case we will drop the directory entry.
+				// This should however not be confused with an object with a trailing slash.
+				dirMatches := best.isDir() == other.isDir()
+				suffixMatches := strings.HasSuffix(best.name, slashSeparator) == strings.HasSuffix(other.name, slashSeparator)
+
+				// Simple case. Both are same type with same suffix.
+				if dirMatches && suffixMatches {
+					toMerge = append(toMerge, otherIdx)
+					continue
+				}
+
+				if !dirMatches && !suffixMatches {
+					// We have an object `name` and a directory `name/`.
+					if other.isDir() {
+						// Discard the directory.
+						if err := selectFrom(otherIdx); err != nil {
+							return err
+						}
+						continue
+					}
+					// Replace directory with object.
+					toMerge = toMerge[:0]
+					best = other
+					bestIdx = otherIdx
+					continue
+				}
+				// Return as separate entries.
 			}
 			if best.name > other.name {
 				toMerge = toMerge[:0]