Moving changes between branches.

[Matt Mackall]

On Tue, 2011-10-11 at 13:38 -0400, Tom Udale wrote:
> Since the diff format used by export is just a standard diff output, it 
> basically descends from RCS/CVS sorts of tools and does not have any 
> idea of multiple parents.

It actually predates RCS (though not SCCS). I've never used SCCS so I
can't tell you what it did instead.

> it still runs into trouble.  One would really need to extend the diff 
> format for DCVS applications to get past this.

It's simultaneously easier and harder than that. We already export the
list of parents in our HG patch header, so we could be using that. But
because export/import is really about -leaving- the system and coming
back into a possibly different system in a different location and merge
is about tying up pairs of loose ends, finding -which- pair of loose
ends the merge is intended to tie up is non-trivial. For instance, if
you ported a and b to multiple branches, now we have to guess which b'
is meant.

But we could also handle this by adding a --second-parent switch to let
you do it manual. No one's cared enough to do that so it hasn't
happened.

-- 
Mathematics is the supreme nostalgia of our time.