Correct some lint console renderer issues with files missing trailing newlines

Summary:
See PHI162. This corrects a couple more bugs:

• If the old file didn't end in a newline, we could end up printing two lines next to each other in the output.
• If the patch targeted "Line 6, character 1" instead of "line 5, character 3" in a file "1\n2\n3\n4\n5\n", we would fail to figure out what that meant when computing an offset because the last line has 0 characters on it.

Test Plan: Added failing unit tests, made them pass. Also tested with some fake linters similar to the ones described in PHI162.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18716