convmv: make --dry-run logs less noisy

Before this change, convmv dry runs would log a SkipDestructive message for
every single object, even objects that would not really be moved during a real
run. This made it quite difficult to tell what would actually happen during the
real run. This change fixes that by returning silently in such cases (as would
happen during a real run.)

fs/operations/operations.go

@@ -428,6 +428,10 @@ func move(ctx context.Context, fdst fs.Fs, dst fs.Object, remote string, src fs.
 	origRemote := remote // avoid double-transform on fallback to copy
 	remote = transform.Path(ctx, remote, false)
 	ci := fs.GetConfig(ctx)
+	newDst = dst
+	if ci.DryRun && dst != nil && SameObject(src, dst) && src.Remote() == transform.Path(ctx, dst.Remote(), false) {
+		return // avoid SkipDestructive log for objects that won't really be moved
+	}
 	var tr *accounting.Transfer
 	if isTransfer {
 		tr = accounting.Stats(ctx).NewTransfer(src, fdst)
@@ -440,8 +444,11 @@ func move(ctx context.Context, fdst fs.Fs, dst fs.Object, remote string, src fs.
 		}
 		tr.Done(ctx, err)
 	}()
-	newDst = dst
-	if SkipDestructive(ctx, src, "move") {
+	action := "move"
+	if remote != src.Remote() {
+		action += " to " + remote
+	}
+	if SkipDestructive(ctx, src, action) {
 		in := tr.Account(ctx, nil)
 		in.DryRun(src.Size())
 		return newDst, nil