Multiple TODOs have been addressed in _is_modified, so that tools

commenting on multiple lines have modifications properly checked. The

algorithm has been made more efficient, only checking relevant

(modified) chunks to see if they are in the range of modification.
A patch was also made to tools that 'globally comment' (I.E. make a

comment with a line of <=0). Previously, all this would do is round the

line number up to 1, (as the front-end does not support rendering

first_line=None), which had the unintended side-effect of tools

configured to only comment on modified code who make global comments, to

only have their comment go through if line 1 was modified. The

first_line=1 workaround is still used, and the file is assumed to be

modified.

I've tested _is_modified through my nyc plugin directly, and

tested File.comment() manually through a stub-plugin. Waiting on test

suite creation to mock up standalone tests.