graphlog: split the actual DAG grapher out into a separate method
This allows extensions like pbranch to use asciigraph() to graph dependencies
for patch branches, but could be used for basically any DAG.
% no diff if hardlink
% init
adding foo to series file
applying foo
Now at: foo
adding bar to series file
applying bar
Now at: bar
430ed4828a74fa4047bc816a25500f7472ab4bfe:foo
4e7abb4840c46a910f6d7b4d3c3fc7e5209e684c:bar
%
foo
bar
%%%
430ed4828a74fa4047bc816a25500f7472ab4bfe:foo
%
foo
%%%
4e7abb4840c46a910f6d7b4d3c3fc7e5209e684c foo
430ed4828a74fa4047bc816a25500f7472ab4bfe bar
%
4e7abb4840c46a910f6d7b4d3c3fc7e5209e684c lfoo
430ed4828a74fa4047bc816a25500f7472ab4bfe lbar
%%%
4e7abb4840c46a910f6d7b4d3c3fc7e5209e684c foo
%
4e7abb4840c46a910f6d7b4d3c3fc7e5209e684c lfoo