Delete license headers from files

Summary:
This commit doesn't change license of any file. It just makes the license implicit (inherited from LICENSE file in the root directory).

We are removing the headers for these reasons:

- It wastes space in editors, less code is visible in editor upon opening a file.
- It brings noise to diff of the first change of any file every year.
- It confuses Git file copy detection when creating small files.
- We don't have an explicit license header in other files (JS, CSS, images, documentation).
- Using license header in every file is not obligatory: http://www.apache.org/dev/apply-license.html#new.

This change is approved by Alma Chao (Lead Open Source and IP Counsel at Facebook).

Test Plan: Verified that the license survived only in unit tests and LICENSE file.

Reviewers: epriestley, btrahan, edward

Reviewed By: epriestley

CC: aran, Korvin, davidrecordon

Maniphest Tasks: T2035

Differential Revision: https://secure.phabricator.com/D3881
Change severity of PEP8 errors to warnings

Summary: None of these are that serious that I would like to be informed about them on unmodified lines.

Test Plan: Linted Python file with lots of PEP8 errors, now warnings.

Reviewers: zeeg, epriestley

Reviewed By: zeeg

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3884
Fix ArcanistLinterTestCase

Summary: We could also inject the value from the test case config but this is simpler.

Test Plan:
  $ arc unit src/lint/linter/ArcanistLicenseLinter.php

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2035

Differential Revision: https://secure.phabricator.com/D3895
Trigger bad charset lint warning only once per line

Summary:
Makes lots of noise:
{F22758}

Test Plan:
Linted file with several bad characters per line.
Linted file with one bad character per line.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3896
Move conversion to dictionary inside lint message

Summary: Maybe I will need it on other places.

Test Plan:
  $ arc lint --output json # on file with lint error

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3898
Ignore duplicate PEP8 lint errors in comprehensive engine

Test Plan:
  $ arc lint a.py # with too long line

Reviewers: zeeg, epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3882
Fix an issue where `arc diff` would loop indefinitely for a property change

Summary:
When we hit a diff which is missing file context, we try to pull it synthetically later. This works for moves and copies, but currently fails for property changes. Since it failed, we didn't have context, so we'd try to pull it again...

The general problem this creates is that when you mark a file "+x" without changing it, we can't show you the content in Differential. Not a huge deal. In some future diff, I'll build the content synthetically.

Adds commits to cover this behavior:

  commit 1830a13adf764b55743f7edc6066451898d8ffa4
  Author: epriestley <git@epriestley.com>
  Date:   Tue Nov 6 17:11:18 2012 -0800

      Mark koan2 +x and edit it.

  commit 8ecc728bcc9b482a9a91527ea471b04fc1a025cf
  Author: epriestley <git@epriestley.com>
  Date:   Tue Nov 6 17:08:44 2012 -0800

      Move 'text' to 'executable' and mark it +x.

  commit 39c8e7dd3914edff087a6214f0cd996ad08e5b3d
  Author: epriestley <git@epriestley.com>
  Date:   Tue Nov 6 16:36:59 2012 -0800

      Mark koan as +x.

Test Plan: Ran unit tests. Previously, they looped indefinitely. Now, they pass.

Reviewers: vrana, btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D3909
Unignore changes in .arc/

Summary:
It's inside `.git/` for some time.
It also ignored changes in `test.arc/`.

Test Plan: None.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3900