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