fix/work around https://bugs.launchpad.net/dulwich/+bug/1025886
Upgraded to use latest version of dulwich (0.9.1).
Updated `setup.py` file to reflect requirement on newer version of dulwich package.
git_handler: clean up trailing whitespace
git_handler: fix bugs introduced by 47df57f2bb2b which could never have passed tests
Merge a work-around for a bug in dulwich.

I've been waiting for dulwich upstream to fix this *and* for a test
from domruf that's acceptable. Having gotten neither over a period of
/months/, and having hit the bug myself, I'm moving on and accepting a
patch without tests. This will likely break again, but hopefully
before we'd break it dulwich will be fixed.
overlays: fix incoming support for hg 2.8

This was crafted mostly via a bunch of aimless flailing in the
code. I'm pretty well convinced at this point that the incoming
support needs to be rewritten slightly to behave properly in the new
world order (specifically, the overlayrepo class probably should be
subclassing localrepo, or else more directly reimplementing things
instead of trying to forward methods.)