Use non-sticky range diff when entering commit files panel

We make the name of the GetSelectedRefRangeForDiffFiles very specific on purpose
to make it clear that this is only for switching to diff files, so the
implementations can make assumptions about that (unlike GetSelectedRef, which is
used for different purposes and needs to stay more generic).

pkg/gui/context/reflog_commits_context.go

@@ -71,6 +71,11 @@ func (self *ReflogCommitsContext) GetSelectedRef() types.Ref {
 	return commit
 }
 
+func (self *ReflogCommitsContext) GetSelectedRefRangeForDiffFiles() *types.RefRange {
+	// It doesn't make much sense to show a range diff between two reflog entries.
+	return nil
+}
+
 func (self *ReflogCommitsContext) GetCommits() []*models.Commit {
 	return self.getModel()
 }