Quick topsort optimizations

- topsort of an app does a full analysis of all apps to come up with
  their app-level DAG
- each extra_src_dir has its own analysis run
- each extra_src_dir run is simulated as being an app
- we therefore end up doing a lot of useless analysis for single app
  runs

This patch notes runs with 1 or 0 apps and skips doing the topsort
analysis on them, which speeds things up further.

src/rebar_compiler_dag.erl

@@ -169,6 +169,8 @@ propagate_stamps(G) ->
 %% @doc Return the reverse sorting order to get dep-free apps first.
 %% -- we would usually not need to consider the non-source files for the order to
 %% be complete, but using them doesn't hurt.
+compile_order(_, AppDefs) when length(AppDefs) =< 1 ->
+    [Name || {Name, _Path} <- AppDefs];
 compile_order(G, AppDefs) ->
     Edges = [{V1,V2} || E <- digraph:edges(G),
                         {_,V1,V2,_} <- [digraph:edge(G, E)]],