Prevent overwriting manual changes made to branch

[alecsammon]

We use the repo-file-sync-action for a number of tasks including syncing changes to our linting configuration.

When we make an update to the linting rules then manual changes are often required in addition to the synced file.

Currently if the sync action runs again then it overwrites any changes make in other files, including all the history.

This PR will adjust this, so that the existing PR is updated instead, and the changes added as a new commit.