Newline character is not respected

### **This is autogenerated. Please review and update as needed.**

## Describe the bug
I want to update the default message with a custom message that has newlines but the CLI is not respecting my newline characters. I've tried `` `n, \n, \\n ``

**Command Name**
`az repos pr update
Extension Name: azure-devops. Version: 0.17.0.`

**Errors:**

## To Reproduce:
Steps to reproduce the behavior. Note that argument values have been redacted, as they may contain sensitive information.

- `az repos pr update --id 1234 --merge-commit-message "Merge PR 14: Implement Authentication\nRelated work item: #12345"`

## Expected Behavior
Multiline string when using `az repos pr show --id 1234` in `completionOptions.mergeCommitMessage` field.

## Environment Summary
```
Windows-10-10.0.19041-SP0
Python 3.6.8
Installer: MSI

azure-cli 2.12.0 *

Extensions:
azure-devops 0.17.0

```
## Additional Context