Use graphp/graph#103 to simplify code.

v0.8.0 of the graph library added the ability to store attributes on
vertices/edges, so we can use that rather than having to store a
parallel commit information data structure.  This makes things much
simpler :)

Author: Spencer Rinehart

composer.json

@@ -24,7 +24,7 @@
         "symfony/process": "~2.4",
         "nubs/sensible": "~0.4.0",
         "nubs/which": "~1.0",
-        "clue/graph": "~0.7.1"
+        "clue/graph": "~0.8.0"
     },
     "bin": ["bin/buildRelease"],
     "autoload": {