Don't close revisions on "arc amend"

Summary:
Ref T4603. This workflow predates `arc land` and doesn't make much sense in modern Phabricator/Arcanist. It is surprising that `arc amend` will sometimes close accepted revisions, and we're better at detecting that repositories are tracked, and tracking repositories is easier.

Also fix some inaccuracies and old claims in the documenation and help.

Test Plan: Ran `arc amend`.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aran, epriestley

Maniphest Tasks: T4603

Differential Revision: https://secure.phabricator.com/D8522
Use more modern detection of repositories in "arc close-revision"

Summary:
Fixes T4603. We fire `arc close-revision --finalize` implicitly from `arc land`, which may close a corresponding Differential revision.

We want to close if the repository is not present in Phabricator (i.e., we'll never be able to close in response to the commit message, since we'll never see it). Historically, we used Arcanist Project -> "Tracked" to make this determination. Instead, just check if the working copy is associated with a repository. This is simpler, easier, and works better.

Test Plan: Ran `arc close-revision --finalize`.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aran, epriestley

Maniphest Tasks: T4603

Differential Revision: https://secure.phabricator.com/D8523
Merge branch 'master' of http://github.com/facebook/arcanist