Make "arc land" great again
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.
- 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.
Reviewed By: chad
Differential Revision: https://secure.phabricator.com/D14356