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