Commit notes into refs/notes/hg-<REMOTE>, to be merged later

If multiple fast-import processes are running concurrently, they cannot
commit to a shared ref such as refs/notes/hg without first getting the
correct parent ref. Unfortunately, there is a seemingly-unavoidable race
condition if one attempts to have fast-import "look up" the value of the
shared ref. This patch instead has each remote commit to a private notes
ref which can be merged later using

    git notes --ref=hg merge hg-<REMOTE>

This command could be scheduled to be run after the fast-import via a hook.
Learn which hgsha1 notes to write based on the last written note

This retroactively applies notes in the fetch following a push, or on
fetch for old repositories for which notes have not yet been
written. Notes are still not eagerly written in push.

Removes xfail from test_simple_push_updates_notes.
Only make one call to mq.strip() on conflicted push