Arcanist - normalize question about unstaged files across workflows (diff vs land)

Summary:
Fixes T4163. Re-word the question so its the same as in "arc diff". Draw the line though and don't automagically do anything if the user says "Y", 'cuz amending / adding files last minute in 'arc land' and other workflows is batshit insane. Assuming infinite growth in the future, I think its best to get this language consistent now.

I changed the shouldAmend member variable to a shouldAmend() function with a static inside. Previously shouldAmend was getting set as a side effect and it was kind of weird. I thought maybe it was written this way because the calls to the vcs are a little slow or something. As such, I figured caching it in the static was a good idea? Didn't seem awful but maybe a premature optimization with whatever the performance reality turns out to be.

Also a modest amount of bonus pht.

Test Plan: this very diff. i'm going to arc land it laters too.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: chad, epriestley, Korvin

Maniphest Tasks: T4163

Differential Revision: https://secure.phabricator.com/D8753
Arcanist - fix shouldAmend code

Summary: make it a real member variable. See rARC77a9c1814063 and D8753#33914.

Test Plan: sending up this diff!

Reviewers: shadowhand, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8761
After landing to a repository, hint that it should be updated

Summary: Fixes T4605. Smart waits (see D8782) are presumably good on the balance, but may cause some delays when changes are made to rarely updated repositories. To help mitigate this, have `arc land` hint that a repository has changed.

Test Plan: Will run `arc land`.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4605

Differential Revision: https://secure.phabricator.com/D8783
Show build results in `arc land`

Summary:
Fixes T4809. When landing a revision, check for a (non-manual) buildable of the current diff. If we find one, check its status:

  - If it passed, print out a message to inform the user that we checked.
  - If it failed or is still building, print out details about the issue and require a confirmation to continue.
  - Just ignore other cases.

Test Plan:
  - Ran `arc land` on a revision with no buildable, a passing buildable, a failed buildable, and a building buildable for the current diff.
  - Got sensible output / prompts.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4809

Differential Revision: https://secure.phabricator.com/D8801
Removing the show message code option from the command line args as it no longer exists in pylint