[PATCH V2] revset: define successors revset

[Jun Wu]

Excerpts from Jun Wu's message of 2017-07-11 21:54:36 -0700:
> Excerpts from Matt Harbison's message of 2017-07-12 00:25:47 -0400:
> > This will be more of an issue with precursors(), which I assume will be  
> > defined the same way for consistency.  I tend to use the following to make  
> > sure that conflicts were resolved properly after rebasing:
> > 
> >     $ hg extdiff --patch --hidden -r "precursors($rev)" -r "$rev"
> > 
> > It can be especially handy in a loop when a series is rebased and several  
> > commits need to be reviewed.  Depth=1 probably solves that.  I only  
> > mention it to provide an actual use case.  Hopefully this remains working  
> > one way or the other in 4.3.
> 
> Yes, I was aware of the use-case. We also have internal users wanting to
> compare with a previous version.
> 
> Note: we have decided to use "predecessor" instead of "precursors" [1].
> A "predecessors" revset is planned. However, I think the current obsstore
> API need some change to support depth better. Since I'm also planning on new
> (hash-preserving) obsstore format and API, I don't want to spend too much
> time on things only working for the format obsstore.

correction: s/format/current/

> 
> The successors revset is mainly to be used together with my rebase work,
> where you can use:
> 
>   hg rebase -s 'unstable()-obsolete()' -d 'max(successors(max(roots(ALLSRC) & ::SRC)^)-obsolete()-divergent())'

correction: s/rebase -s/rebase -r/, so does the command below.

> to basically achieve what "evolve --all" can do today, even with more
> flexibility like you can move the destination approximately to a head:
> 
>   hg rebase -s 'unstable()-obsolete()' -d 'max((successors(max(roots(ALLSRC) & ::SRC)^)-obsolete()-divergent())::)'
> 
> (I'd wish that feature to be available in 4.3, but that's just a wish)
> 
> Predecessor revset is not needed for that so it's less prioritized.
> 
> [1]: https://www.mercurial-scm.org/wiki/CEDVocabulary