Minor change to `ArcanistXHPASTLinter`

Summary: Change `==` to `===`. This is a little bit cleaner because we shouldn't need type coercion here.

Test Plan: Ran `arc unit`.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9893
Remove Phabricator reference from the linter description of `ArcanistXHPASTLinter`

Summary: The `ArcanistXHPASTLinter` is no longer //too// specific to Phabricator (Phabricator-specific XHPAST lints generally live in `ArcanistPhutilXHPASTLinter`) and //should// be better suited for general purpose usage.

Test Plan: N/A

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9892
Use `git diff a b` for ranges, not `git diff a..b`

Summary:
These are documented as being identical, but `git diff a b` works if `a` is a tree (for example, `4b825d...`, the empty tree hash), but `git diff a..b` does not.

Particularly, with the `a..b` form, `arc diff --base arc:empty` does not work. With the `a b` form, it does.

Test Plan: Ran `arc diff --base arc:empty` in a repository and got a diff.

Reviewers: btrahan, talshiri

Reviewed By: talshiri

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9898
Patch: don't complain about reviewers not being specified.
This will break in the future when upstream arc incorporates T4631. Hopefully there will be a better solution by then