rsync regression 4.42 -> 4.44

Using the old gsutil, I can run the following commands without issue:

```
mkdir test
touch test/test.txt
gsutil rsync test gs://my-bucket/test-test
gsutil rsync test gs://my-bucket/test
```

The new version fails on the final command with `CommandException: arg (gs://my-bucket/test) does not name a directory, bucket, or bucket subdir.`. Changing the final command fixes it:

```
gsutil rsync test gs://my-bucket/test/
```

Was this change intentional?

Start by reproducing the two gsutil rsync sequences against the affected versions and compare the final destination-argument validation. Trace the rsync command entry point and its directory or bucket-subdirectory checks; done means the intended trailing-slash behavior is documented by a regression test and the compatibility decision is clear.

Written by the indexing model from the issue text.