Better message on branch comparison view

When you change the branch on the dropdown selector the message should change to something close to "Click on the Compare branches and continue button to check changes_"

When we create a merge request from a fork of the master repository, we can select the source and destination branch that we want to merge. Now I'm not sure why, but the box below the selected branches keep stating that there is noting to merge (see screenshot below). I think that gitlab does not do the compare between the branches on the go because of the performance penalty that it would mean but to my opinion it would be better if the box stating that there is nothing to merge was not there at all. The current situation is confusing to our developers.

ZD reference: 20211