Modernize `ArcanistMergeConflictLinter`.

Summary: The only real change here is adding a `getLinterConfigurationName` method so that this linter can be used with an `.arclint` file. Everything else is just some minor tidying.

Test Plan: N/A

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9039