generalize upgrade to take more than one lib pair simultaneously

[mitchellwrosen]

Overview

This PR generalizes upgrade to allow upgrading multiple libraries simultaneously:

> upgrade foo1 foo2 bar1 bar2 baz1 baz2 ...

(it's an error to provide an odd number of arguments).

One small user-visible implementation quirk: if an upgrade fails, we had been putting the user on a branch called upgrade-<old>-to-<new> to resolve it, as a reminder about the branch context. However, I didn't want to continue that template for an arbitrary number of library upgrades, so if a 2+ library upgrade fails, the user is just put on a branch called upgrade (or upgrade__2 if upgrade is taken, etc).

Test coverage

I've added tests to the upgrade.md transcript.

[aryairani]

Nice!

Could you explain the different cases with the different messages; I guess when there's underscores in the arg names, then it doesn't try to "rename", and then under certain circumstances it does?

And then I think we should probably not have a bulleted list with just one bullet.

[mitchellwrosen]

Regarding renaming, it's the same behavior as before - if the library you are upgrading to ends in __N then we try to rip it off afterwards. This is to catch the common case of installing an unpublished library, like:

> lib.install @mitchellwrosen/something
-- ok I installed lib.mitchellwrosen_something_main

> lib.install @mitchellwrosen/something
-- ok I installed lib.mitchellwrseon_something_main__2

> upgrade mitchellwrosen_something_main mitchellwrosen_something_main__2
-- ok I upgraded mitchellwrosen_something_main to mitchellwrosen_something_main__2, then renamed it to mitchellwrosen_something_main

[mitchellwrosen]

And then I think we should probably not have a bulleted list with just one bullet.

Ok, I just replaced it with a comma-separated list then, like in the successful upgrade message