Fix early exit condition

I don't know what this condition is supposed to guard against, or whether we
really need it (it was added in 06ca71e955, and the commit message of that
commit only says "fix bug"). But if we do need it, then it seems that `>=` is
more correct than `>`.

pkg/gui/presentation/commits.go

@@ -65,7 +65,7 @@ func GetCommitListDisplayStrings(
 		return nil
 	}
 
-	if startIdx > len(commits) {
+	if startIdx >= len(commits) {
 		return nil
 	}