[PATCH 2 of 5] convert: support "preprocess" in filemap

[Yury Sulsky]

> Here's the goal: each changeset should make a single, focused, clear,
> easily described change that makes Mercurial unambiguously better. Fix
> one bug. Factor out one module. Fix bad names in one area of the code.
> Add one step of a 7-step major feature. You get the idea.

Thanks very much Greg! I'll rework the patches to be more self-contained.

Two questions, though. You mentioned that I should send the
includerevs changes before the filemap -> filtermap refactor, but this
might cause a bit of (avoidable) code churn. I think the approach
would be to make filtermap a subclass of filemap, and to add the
includerevs changes there. But that's not quite right, because we
would have to pass in a dummy filemapper in the case when
--includerevs is specified, but not --filemap. The next patch would
then fix this. Do you think we should do that, or should I submit the
filemap -> filtermap patch first?

Also, should these patches be in the "default" branch instead of the
"stable" branch?

Yury