Option to not fail if everything is up to date

[skeet70]

It seems like most users (and the intent) of this action is to keep a repository up to date with upstream changes. Currently if the upstream merge fails because everything is already up to date, the action fails. That seems counter to the intended use of the action. It should succeed as no operations were required to get to the desired state.

Your branch is up to date with 'origin/main'.
Error: Process completed with exit code 128.