Don't lint symlinks by default.

Summary: Fixes T5300. Currently, if a dead symbolic link is linted, all kinds of errors will be thrown by most linters because they will try to read the (non-existent) file contents. Instead, let's not lint symbolic links by default. In the case that the target of a symbolic link is inside the working copy, then it should be being linted anyway.

Test Plan: Created a symbolic link and verified that it wasn't linted (by any linter other than the `ArcanistFilenameLinter`).

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5300

Differential Revision: https://secure.phabricator.com/D9448