Disable kopia optimization that lets us reuse a base directory (#2522)

## Description

Just handing the base directory to kopia doesn't let us see which items have been added which means we can't properly merge backup details later on.

Future implementations could do backup details merging based on path prefixes which could fix this problem.

## Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No 

## Type of change

- [ ] 🌻 Feature
- [x] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Test
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup

## Issue(s)

* closes #2521

## Test Plan

- [ ] 💪 Manual
- [x] ⚡ Unit test
- [ ] 💚 E2E

src/internal/kopia/upload.go

@@ -526,23 +526,19 @@ func buildKopiaDirs(
 	globalExcludeSet map[string]struct{},
 	progress *corsoProgress,
 ) (fs.Directory, error) {
-	// Reuse kopia directories directly if the subtree rooted at them is
-	// unchanged.
-	//
-	// TODO(ashmrtn): We could possibly also use this optimization if we know that
-	// the collection has no items in it. In that case though, we may need to take
-	// extra care to ensure the name of the directory is properly represented. For
-	// example, a directory that has been renamed but with no additional items may
-	// not be able to directly use kopia's version of the directory due to the
-	// rename.
-	if dir.collection == nil && len(dir.childDirs) == 0 && dir.baseDir != nil && len(globalExcludeSet) == 0 {
-		return dir.baseDir, nil
-	}
-
 	// Need to build the directory tree from the leaves up because intermediate
 	// directories need to have all their entries at creation time.
 	var childDirs []fs.Entry
 
+	// TODO(ashmrtn): Reuse kopia directories directly if the subtree rooted at
+	// them is unchanged.
+	//
+	// This has a few restrictions though:
+	//   * if we allow for moved folders, we need to make sure we update folder
+	//     names properly
+	//   * we need some way to know what items need to be pulled from the base
+	//     backup's backup details
+
 	for childName, childDir := range dir.childDirs {
 		child, err := buildKopiaDirs(childName, childDir, globalExcludeSet, progress)
 		if err != nil {