[PATCH v2] pager: migrate heavily-used extension into core

[Jun Wu]

Excerpts from Augie Fackler's message of 2017-02-06 17:52:30 -0500:
> (sending again because this seems to have gotten stuck somewhere...)
> (+yuya explicitly for chg thoughts, +smf so someone else can forward to the list if it's stuck again)

chg would be fine so long as ui.pager() calls ui._runpager().

> [snip]
> How does that sound, overall? I'll tackle the refactoring today or
> Wednesday to mail an RFC patch if that doesn't sound too much like the
> perfect being the enemy of the good.
> 
> (This design, btw, was inspired by the way git handles paging, where
> it's largely up to the command to decide if it wants to invoke the
> pager.)

The API is not new to me, I'm +1 on it, and did the chg ui._runpager
refactoring to make the change easier.

The only concern I have is the amount of work actually needed to finish the
change. It probably has to be a separate series.
 
> As a sketch of where this is headed, API-wise:
> 
> class ui:
>  def pager(self, command, category):

Just to confirm if I understand correctly, command and category are only
used to find out the actual pager command from the config? That looks good.
Maybe swap category and command and make command (or both) optional, to
discourage using commands in configs.