Emit full URIs to identify Differential revisions

Summary:

• Previously, used IDs like "33" to match a commit to a Differential revision.

This has a namespacing problem because we now have an arbitrarily large number
of Phabricator installs in the world, and they may want to track commits from
other installs.

• In Differential, parse raw IDs or full URIs. Emit only full URIs.
• In Repositories, parse only full URIs.
• This might cause a few commits to not be picked up in rare circumstances.

Users can fix them with "arc mark-committed". This should be exceedingly rare
because of hash matching.

• There are some caveats for reparsing older repositories, see comments

inline. I don't think there's much broad impact here.

Test Plan:

• Created a new revision, got a full URI.
• Updated revision, worked correctly.
• Ran unit tests.
• Monkeyed with "Differential Revision" field.
• Reviewers: btrahan, jungejason

Reviewers: btrahan, jungejason

Reviewed By: jungejason

CC: aran, epriestley, jungejason

Maniphest Tasks: T54, T692

Differential Revision: 1250