Make "arc land" great again

Summary:
Ref T3855. Fixes T9537. Fixes T8620. Fixes T4333.

This declares bankruptcy and replaces the entire arc land workflow under Git. These are the notable changes:

• (T3855) You can now land from a branch to itself.
• (T3855) We now try to restore the original state very aggressively after any failure, instead of dumping you into the middle of a mess.
• (T9537) You can now land without a local branch.
• ([not actually] T9543) We'll now ignore the local branch if it just happens to be named the same thing as the remote branch but doesn't actually track it.
• (T8620) You can now land from a detached HEAD.
• (T4333) We now preserve the author and author date of whatever you land.

This may need some followup work. In particular:

• The signal handler (that tries to put you in a better place if you ^C in the middle of things) causes ^C to work awkwardly in prompts. This might not be worth it.
• Errors/instructions on push/merge issues might need work.
• I dropped support for --delete-remote and --update-with-blah-blah because I think these flags aren't worth their complexity.
• I've simplified the update/merge algorithm a bit. It may need some complexity added back in.
• I probably missed a few things because this covers like 200 unique, creative workflows.
• Users might need more guidance on the workflows that drop them in the middle of nowhere if they manage to reach them more often than I think.

Test Plan:

• Used arc land to land like at least 15,000 different kinds of changes.
• Landed normally.
• Landed from a branch onto itself.
• Landed from a detached head.
• Landed nothing.
• Landed with no local branch.
• Landed onto made-up branches.
• Landed with bad targets.
• ^C'd things in the middle.

Reviewers: chad

Reviewed By: chad

Subscribers: tycho.tatitscheff

Maniphest Tasks: T3855, T4333, T8620, T9537, T9543

Differential Revision: https://secure.phabricator.com/D14356