In arc diff, try to guess where a change should land

Summary:
Ref T9952. Ref T3462. My primary goal is to improve prefilling of the "Onto Branch:" field in the "Land Revision" dialog.

When uploading a diff with arc diff, add a property with some information about which branch to target. In particular:

• If the local branch tracks an upstream branch (or tracks something which tracks something which tracks the upstream), target that.
• If not, but "arc.land.onto.default" is set, target that.

This doesn't try to guess in other cases, since they're more involved. I'll add some context about this in T3462.

I don't love using "diff properties" for this, but it doesn't make cleaning them up any harder since we already use it for other stuff which isn't going away (lint/unit excuses).

Test Plan:

• Added some var_dump() and used arc diff --only to generate diffs.
• Saw upstream tracking and config-based rules generate reasonable values and submit them.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T3462, T9952

Differential Revision: https://secure.phabricator.com/D14736