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
Tidying up of linter code.

Summary: Various tidying up of linting code.

Test Plan: `arc lint` and `arc unit` still pass.

Reviewers: chad, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9625
Update callsites of `phutil_json_decode`.

Summary: Depends on D9634. `phutil_json_decode` now throws an exception on invalid JSON.

Test Plan: Ran the unit tests.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9641
Don't throw an exception if no configuration is set for linter tests

Summary: This was broken in D9641. If `$config` is not set, then a `PhutilJSONParserException` will be thrown. The expected behaviour is that it is okay to not explicitly set any configuration.

Test Plan: Ran `arc unit`.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9643