Potential BC changes to sparse before freeze

[Jun Wu]

Excerpts from Gregory Szorc's message of 2017-07-11 12:50:02 -0700:
> Without typing up a formal proposal, I'd like to reuse the low-level config
> parser - not the higher-level config layer baked into ui.py. I /think/ that
> prevents things like aliases from creeping in. I'd also like to think about

I think even the low-level config parser might be over complicated for the
sparse usecase. Like its support of "%unset", order preserving, multiline
handling, and including relative paths etc.

If we are going to unify the file formats, I wonder if it's possible to be
built on top of hgignore format. They seem to be more similar, and is
simpler than the config parser. And if we made optimizations to regular
expressions for tree manifest hgignore in the future, a same format will
ensure the optimizations could be trivially reused in sparse too.

> how to use e.g. a .hgsparseprofiles file for centrally defining some
> profiles. Right now you have to know the path to a profile in the repo.
> This means repos can't do nice things like automatically advertise common
> profiles for users to use. If you have a central file defining multiple
> profiles, then you need sections. And all of a sudden a custom config
> format and parser seems a bit redundant.
> 
> I'll try to come up with a more formal proposal...
> 
> Thanks for the feedback!