graph: add outputgraph() function, called by ascii() to print the graph to the ui. This allows a cleaner entrypoint for extensions to tweak the graph output without needing to rewrite all of ascii(), or needing to manually guess where the graph nodes/edges end and the rev note portion begins. This patch does not affect graph output or behavior in any way. Differential Revision: https://phab.mercurial-scm.org/D3655