Merge strategy option for pull request create

`az repos pr create` does not have an option for merge strategy, and by default is created with:

```
"completionOptions": {
...
"mergeStrategy": "noFastForward",
...
},
```

This leads to an error that when `--auto-complete` is specified, the PR will never be completed.

---
#### Document Details

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* ID: 3324c35b-f381-8db4-b168-6f73e20c1f3e
* Version Independent ID: 7a3e8656-7741-8365-e56c-44056f034b31
* Content: [az repos pr](https://docs.microsoft.com/en-us/cli/azure/ext/azure-devops/repos/pr?view=azure-cli-latest#ext-azure-devops-az-repos-pr-create)
* Content Source: [latest/docs-ref-autogen/ext/azure-devops/repos/pr.yml](https://github.com/MicrosoftDocs/azure-docs-cli/blob/live/latest/docs-ref-autogen/ext/azure-devops/repos/pr.yml)
* GitHub Login: @rloutlaw
* Microsoft Alias: **routlaw**