Don't append empty comp.append

In some cases, comp.append could be an empty string. This would happen
if a parser was something like `(token(foo) <~ ;).+ <~ fo.?` because there were
no completions for the `fo` available anchor. The effect of this was
that tab would never complete foo;f to foo;foo, even though that was the
only possible completion. It would, _display_, foo as a possible
completion though.

This came up because the multi parser has a similar parser to that
described above and it broke tab completion to the right of a semi
colon.

internal/util-complete/src/main/scala/sbt/internal/util/complete/JLineCompletion.scala

@@ -83,7 +83,8 @@ object JLineCompletion {
     val (insert, display) =
       ((Set.empty[String], Set.empty[String]) /: cs) {
         case (t @ (insert, display), comp) =>
-          if (comp.isEmpty) t else (insert + comp.append, appendNonEmpty(display, comp.display))
+          if (comp.isEmpty) t
+          else (appendNonEmpty(insert, comp.append), appendNonEmpty(display, comp.display))
       }
     (insert.toSeq, display.toSeq.sorted)
   }