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.
% add file
adding qqq.txt
% commit first revision
% set bookmark
% commit second revision
% set bookmark
% update to -2
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
% commit new head
created new head
% bookmarks updated?
test 1:16b24da7e457
test2 1:16b24da7e457
% strip to revision 1
saving bundle to
saving bundle to
adding branch
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
% list bookmarks
* test 1:9f1b7e78eff8
* test2 1:9f1b7e78eff8