remove 0-item progress bars (#1986)

## Description

Incremental backups can display progress bars
with 0 items to add or remove.  This can cause
end-user confusion when hte bar completes
with 0 items involved.  We should instead keep
those bars hidden.

Also removes the "heuristics" progress step.

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

- [x] ⛔ No 

## Type of change

- [ ] 🐹 Trivial/Minor

## Issue(s)

* #1984

## Test Plan

- [x] 💪 Manual

src/internal/connector/exchange/exchange_data_collection.go

@@ -176,22 +176,30 @@ func (col Collection) DoNotMergeItems() bool {
 // all the M365IDs defined in the added field. data channel is closed by this function
 func (col *Collection) streamItems(ctx context.Context) {
 	var (
-		errs       error
+		errs        error
-		success    int64
+		success     int64
-		totalBytes int64
+		totalBytes  int64
-		wg         sync.WaitGroup
+		wg          sync.WaitGroup
+		colProgress chan<- struct{}
 
 		user = col.user
 	)
 
-	colProgress, closer := observe.CollectionProgress(user, col.fullPath.Category().String(), col.fullPath.Folder())
-	go closer()
-
 	defer func() {
-		close(colProgress)
 		col.finishPopulation(ctx, int(success), totalBytes, errs)
 	}()
 
+	if len(col.added)+len(col.removed) > 0 {
+		var closer func()
+		colProgress, closer = observe.CollectionProgress(user, col.fullPath.Category().String(), col.fullPath.Folder())
+
+		go closer()
+
+		defer func() {
+			close(colProgress)
+		}()
+	}
+
 	// get QueryBasedonIdentifier
 	// verify that it is the correct type in called function
 	// serializationFunction
@@ -228,7 +236,9 @@ func (col *Collection) streamItems(ctx context.Context) {
 			atomic.AddInt64(&success, 1)
 			atomic.AddInt64(&totalBytes, 0)
 
-			colProgress <- struct{}{}
+			if colProgress != nil {
+				colProgress <- struct{}{}
+			}
 		}(id)
 	}
 
@@ -282,7 +292,9 @@ func (col *Collection) streamItems(ctx context.Context) {
 			atomic.AddInt64(&success, 1)
 			atomic.AddInt64(&totalBytes, int64(byteCount))
 
-			colProgress <- struct{}{}
+			if colProgress != nil {
+				colProgress <- struct{}{}
+			}
 		}(id)
 	}
 

src/internal/observe/observe.go

@@ -344,6 +344,7 @@ func CollectionProgress(user, category, dirName string) (chan<- struct{}, func()
 			decor.CurrentNoUnit("%d - ", decor.WCSyncSpace),
 			decor.Name(fmt.Sprintf("%s - %s", user, dirName)),
 		),
+		mpb.BarFillerOnComplete(spinFrames[0]),
 	}
 
 	if !cfg.keepBarsAfterComplete {

src/internal/operations/backup.go

@@ -255,13 +255,6 @@ func produceManifestsAndMetadata(
 	tenantID string,
 	getMetadata bool,
 ) ([]*kopia.ManifestEntry, []data.Collection, error) {
-	complete, closer := observe.MessageWithCompletion("Fetching backup heuristics:")
-	defer func() {
-		complete <- struct{}{}
-		close(complete)
-		closer()
-	}()
-
 	var (
 		metadataFiles = graph.AllMetadataFileNames()
 		collections   []data.Collection