Turn if/else branch for valid compare error message

##### What does this MR do?

This PR switches the error messages, if compare logic can't provide a comparison result.

##### Why was this MR needed?

If we're comparing the same revision, UI says `You'll need to use different branch names to get a valid comparison.`. If it can't compare for some reason (e.g. branch doesn't exist), it says `branch1 and master are the same`.

##### What are the relevant issue numbers?

This is related to #7446