Tracking 3rd-party sources

[Marcin Kasperski]

> now you work on active.  when an update comes out you unpack
> into proj-pristine, hg addremove ; hg ci.  then you do hg pull
> inside of proj-active.  this only pulls the CHANGES from the
> old to new version.  you do NOT have worry about re-applying
> your patch in any way, 

My patch is already in... So I have to worry.

How should be - in this scenario - handled the case, when I am to 
abandon some of my changes altogether? Say, mainstream 
implemented some feature which I used to implement myself (in 
the same, or other way), or mainstream changed itself so that my 
changes are no longer usable?

Such problem can manifest itself as conflict during the pull 
(touching all the files affected, or only part of them), or can 
not manifest at all.