stop release-please from blowing away manual edits

**Is your feature request related to a problem? Please describe.**

Sometimes a maintainer may need to edit a release PR in order to provide extra context, deprecation/migration warnings, etc to the release notes. Currently if a person does this and another commits comes in after the changes were made and before the PR was merged those manual edits get blown away.

**Describe the solution you'd like**

It would be nice is release-please would honor a Github label and no longer push to the branch.

**Describe alternatives you've considered**

Another approach would be to have a command comment to due something similar. Example: `/stopUpdates`

**Extra context**

Either way in this case I think release please may need to keep track of what commit it should tag at.