fix counting of context lines (don't count lines w/no chars included)

[sqs]

In the context summary in chat, ✨ Context: 56 lines from 5 files previously counted lines even if no characters on that line were included. For example, a range from line 5 character 10 to line 7 character 0, which is rendered as 5-6, was counted as 3 lines. It should be counted as 2 lines.

Test plan

CI

[sqs]

Nevermind. Will land as part of #3287. I initially tried to extract this from #3287 here, but I found that there were other bugs elsewhere in the code (a few different places where we parsed the range strings differently) that meant it would be a more involved change.