Changes to Arcanist for libphutil "extensions/"

Summary: See D5714. Ref T2971.

Test Plan: Built a library map for libphutil's test library.

Reviewers: vrana, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2971

Differential Revision: https://secure.phabricator.com/D5715
Adding `--` to the end of git log so can `arc land` when the branch is
named the same as a file path.

Summary: (In fbcode and www at least, the only places I tried) if a
branch happens to be named the same as a file path (from root) (like if
a branch is called `spec` and there is a directory in `www` called
`spec`) then `arc land` will fail with git complaining that the argument
(to `git log ...`) is ambiguous as it could refer to both a path and a
revision; so this diff adds a `--` to the end so that git knows that
both are revisions and not paths.

Test Plan: Try and land something from www (I did, see D752219) where
the branch is named `spec`.

Reviewed by: epriestley
Disable lint cache by default

Summary:
Fixes T2266. Motivation:

  - The lint cache does not always invalidate correctly. Because of the nature of the cache, this is a hard problem (right after "naming things").
    - We already have a fair amount of complexity in trying to invalidate it, and are still discovering new places where it doesn't work (e.g., Windows with "/" vs "\" paths).
    - One invalidation failure is when linter code changes, which seems unresolvable in the general case (e.g., changes to external linters).
  - It's not obvious what's happening when the lint cache causes some kind of issue.
    - Particularly while developing or debugging linters, your changes often won't be reflected in the lint output. Some of this is theoretically tractable but the external linter case probably isn't.
  - When someone reports a problem with the lint cache in IRC or elsewhere, there is essentially never a way for me to fix it. The lint cache can't be debugged effectively without access to a working copy where the problem reproduces.
  - The cache provides limited benefit outside of Facebook's install.

To remedy these issues:

  - Introduce configuration which controls cache usage.
  - Default it off.
  - Print a message when the cache is in use.

(I'd tentatively support removing the cache entirely, but I don't know how @vrana and Facebook feel about that.)

Test Plan: Ran `arc set-config --show`, `arc lint --cache 0`, `arc lint --cache 1`.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran, mbishopim3, nh, edward, wez

Maniphest Tasks: T2266

Differential Revision: https://secure.phabricator.com/D5766
Merge branch 'master' of http://github.com/facebook/arcanist