regression: rsync -r fails if destination folder has no trailing slash

## gcloud info

```
Google Cloud SDK [281.0.0]

Platform: [Windows, x86_64] ('Windows', 'DESKTOP-E9T5K4M', '10', '10.0.18362', 'AMD64', 'Intel64 Family 6 Model 158 Stepping 10, GenuineIntel')
Locale: ('en_US', 'cp65001')
Python Version: [2.7.13 (v2.7.13:a06454b1afa1, Dec 17 2016, 20:53:40) [MSC v.1500 64 bit (AMD64)]]
Python Location: [C:\Users\danie\AppData\Local\Google\Cloud SDK\google-cloud-sdk\platform\bundledpython\python.exe]
Site Packages: [Disabled]
```

## Issue

I'm running into an issue when running the following command:
`gsutil -m rsync -r build/ gs://aci-front-releases/v1.2.0`

It fails with the following output:
`CommandException: arg (gs://aci-front-releases/v1.2.0) does not name a directory, bucket, or bucket subdir.`

Adding a trailing slash to the destination works:
`gsutil -m rsync -r build/ gs://aci-front-releases/v1.2.0/`

I think this is a regression because I had a cloud builder configuration that was running this exact command and had always succeeded until recently.

Here is my cloudbuild configuration file:

```
steps:
- name: 'node:12'
entrypoint: npm
args: ['ci']
- name: 'node:12'
env:
- NODE_ENV=production
entrypoint: npm
args: ['run', 'build']
- name: 'gcr.io/cloud-builders/gsutil'
args:
[
'-m',
'rsync',
'-r',
'-d',
'build/',
'gs://aci-front-releases/$BRANCH_NAME',
]
```

Start with the gsutil rsync command shown in the issue and reproduce it using a local build/ directory and the Google Cloud Storage destination without a trailing slash. Compare it with the working trailing-slash form; done means the original command succeeds without requiring that slash.

Written by the indexing model from the issue text.