discovery: improve partial discovery documentation The older documentation could be improved.

discovery: add a simple `addinfo` method The method can directly process a sample result. This makes the main code simpler to follow.

discovery: move missing tracking inside the partialdiscovery object This is the final set that we need to track to have a fully up to date information within the object.

discovery: add a `iscomplete` method to the `partialdiscovery` object The method is used by higher level logic to know if the initial discovery question has been answered.

discovery: update undecided from common directly within partialdiscovery The object knows when there are changes in the "common" set, so it is a good place to update the undecided set.

discovery: move undecided set on the partialdiscovery To initialize it, we need to know the discovery target. This commit only move the set on the `partialdiscovery` object, later changeset will take advantage of it.

histedit: use context manager for locks Differential Revision: https://phab.mercurial-scm.org/D5542

histedit: drop unused constructor arguments (API) All in-tree callers pass a repo and nothing else. Differential Revision: https://phab.mercurial-scm.org/D5541

phabricator: teach {phabreview} to work without --amend There's some possibility of this being wrong (e.g. if the url config was changed). But commit messages can be amended too, so that isn't bulletproof either. Having something seems better than just returning None.

phabricator: warn if unable to amend, instead of aborting after posting There was a divergence in behavior here between obsolete and strip based amending. I first noticed the abort when testing outside of the test harness, but then had trouble recreating it here after reverting the code changes. It turns out, strip based amend was successfully amending the public commit after it was posted! It looks like the protection is in the `commit --amend` command, not in the underlying code that it calls. I considered doing a preflight check and aborting. But the locks are only acquired at the end, if amending, and this is too large a section of code to be wrapped in a maybe-it's-held-or-not context manager for my tastes. Additionally, some people do post-push reviews, and amending is the default behavior, so they shouldn't see a misleading error message. The lack of a 'Differential Revision' entry in the commit message breaks a {phabreview} test, so it had to be partially conditionalized.