perf scripts python: call-graph-from-sql.py: Factor out CallGraphModel from TreeModel

Factor out CallGraphModel from TreeModel, which paves the way to reuse
TreeModel in future reports.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20181001062853.28285-10-adrian.hunter@intel.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>