Support for filtering incremental streams

[Alex Usov]

filter_processor is quite aggressive right now: it will unconditionally squash empty commits and rewrite parents.

It is not currently usable for the simple cases, like removing a file or two from the the stream:
 - if you run it on a partial (branch only) stream it will remove "from" references to the parent tree
 - and if you run it on a whole history of the branch it will squash empty commits from shared history

After some poking around the code it seems it should be relatively easy to add extra flag (or deduce flag value from includes/excludes) to make it more conservative, so that it will preserve all empty commits & unresolved "from" links.

Are there any underwater stones on this path?