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