[PATCH 9 of 9] template: use template-engine for obsfate

[Jun Wu]

Excerpts from Boris Feld's message of 2017-07-13 12:32:39 +0200:
> I think there was a misunderstanding about obsfate that I will try to
> clarify, this email is a bit long, sorry.
> 
> TL;DR:
> - hg log is a changeset-centric command
> - obsfate template summarize the obs-history between a changeset and
> its successors, history that can span several obs-markers

I agree a function is needed to work with multiple obsmarkers. But I don't
think the current obsfate implementation is a good design.

I think that function's input is (src, [dest=(not hidden())]), output is
multiple obsmarkers. The user could choose what to do with a list of ordered
obsmarkers. Like, if they want to display the entire chain of markers, they
should be able to do it. If they want to summarize things, we could provide
"dedup", "if_unique" helper function to allow them to do so.

> - hg obslog from evolve extensions is a obs-marker centric command that
> could gain support for operation

It still prints changesets, but uses a different DAG. It could be a --dag
flag of "hg log" command. We might also want to add other DAG types, like a
linear (ignore p2) graph [1].

[1]: https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-July/101148.html

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