We've had a long-standing TODO item for optimizing how line matching

works. We previously had to do a full row scan of all chunks of a diff

in order to find a line, for each line being commented on. This was

slow, and it was often done several times per diff.
The logic for the scanning was also repeated in various ways a few

times, and was about to be repeated again.
Part of this change redoes this logic, giving us a single function for

iterating through lines, with an optional start line. That start line is

found through a binary search of chunks, and a relative offset into the

matching chunk's lines.
The other parts introduce a new method and a fix to an existing method.
The new get_lines() can be used to fetch a range of lines from the

original or modified file. This will be useful for the shellcheck tool

in an upcoming change.
The _is_modified() function has been revised to consider a range of

lines to be modified if any lines within it are modified. Previously,

the entire range had to be considered modified, and this could lead to

tools choosing not to comment on a line because some part of the range

wasn't modified.

All unit tests pass on Python 2.7 and 3.x.
Tested this code along with some new logic coming to shellcheck.