Managing long patch series

[Jim Blandy <jimb@codesourcery.com>]

Ulrich, I'm curious what techniques you use to manage these long
strings of patches.  Specifically, I was wondering:

- I'm usually working from a fully-patched tree, and then breaking it
  up into digestible pieces for submission.  If you are working this
  way as well, do you have a nice way to ensure the decomposed patch
  series remains equivalent to your fully-patched tree?

- Often I find I need to revise an earlier patch in the series, but
  that chance may affect later patches.  Do you have a nice way to
  handle this?

Or is it all just "blood, sweat, and tears"?  In the software world,
that approach usually results in "mistakes", but you and your fellow
IBM GDB hackers seem to do well.

I've tried using quilt, but if one doesn't keep very careful track of
what's going on things can get very tangled.  The Emacs mode helped
somewhat, but had other flaws, so I set it aside.

I've been tempted to try using Mercurial for this.