Do not disable the Changes button when not necessary

1. Log window: Select two commits (click, then ctrl+click).

2. The Changes button in the toolbar (F4) is disabled. (Why? Am I prohibited to make a diff at this moment? This is illogical.)

3. Click anywhere inside of one of the diff widgets. Now the Changes button becomes active. (It should have been active in step 2.)

Note: The button tooltip reads: "Show local changes. F4" I think the word local is superfluous here. We can't show remote changes anyway, any remote data are first downloaded to the local computer by git. Does it mean "Show changes to the working directory?" No, it can be used to show a diff between any two commits. I think "Show changes. F4" or "Show a diff. F4" should be enough.