multiple `cml pr` runs

If the data is changed after a PR it's going to run again generating strange branch names.

```
experiment-cml-pr-da72cf16-cml-pr-da72cf16-cml-pr-da72cf16
```

Things to consider:
- If we revert the state use ```send-comment``` to comment into the new PR via ```cml send-comment --pr --commit-sha HEAD``` wont work
- Do not take into account current branch name to avoid concatenation
- Take into account file changes to generate the branch name also we have to consider #792

I wonder if is practical to fix it, I mean, should ```cml pr``` allow double execution? Which ones would be those plausible scenarios?