The short version:
 * Patches should have a good summary line for first line of commit message
 * Patches should be sent to the Google Group[0]
 * Patch needs to do exactly one thing
 * testsuite passes

The longer version:

We use a variant of Mercurial's own contribution
system. https://www.mercurial-scm.org/wiki/ContributingChanges contains
their full guidelines. Key differences are (by rule number):

 1. For hg-git, I'm not strict about the "topic: a few words" format
    for the first line, but I do insist on a sensible summary as the
    first line of your commit
 2. We don't require use of issueNNN for bug fixes (we have no robot)
 3. Use the hg-git google group
 10. We use mostly pep8 style. The current codebase is a mess, but new
     code should be basically pep8.

0:
Mailing list: hg-git@googlegroups.com
Archives: https://groups.google.com/forum/#!forum/hg-git

Pull requests are generally not noticed more than once every few
months. If you do a pull request, I'm still going to expect you to
have a clean history, and to be willing to rework history so it's
clean before I push the "merge" button.