Add unit tests for ArcanistMergeConflictLinter.

Summary: Self explanatory. Also fixed the character offset to start at 1 instead of 0 (because the other linters seem to do this).

Test Plan: arc unit

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9626