Fix interdiffs for small files or changes near the top of the file.
The interdiff filtering code made some assumptions about the number of

lines of context around changes in a diff chunk. This was believed to be

mostly valid for a typical diff, but that was a bad assumption.
Any changes near the top of the file (specifically, within those 3

assumed lines of context) would be filtered out. If the file was fewer

than 3 lines, it would flat-out break, due to an assumption of a length

indicator in the chunk header that was missing. Even fixing that, since

the file's total number of lines fell within the 3 lines of assumed

context, the whole file would appear unchanged.
Instead of assuming things, we now do a little more processing on the

diff. We go through each line and look for the chunk headers, as we did

before (previously using finditer). When we find one, we begin looking

at the following lines for any line change indicator, and we count how

many lines are unchanged. We can then use that to more accurately

determine the context of the chunk.

Tested with an interdiff for a 1-line file, which previously resulted in an error.

Instead, I saw the interdiff with the appropriate highlighting.
Tested with an interdiff changing one of the first lines in the file. It too passed.
Tested with other interdiffs, and didn't see any problems.
Unit tests pass.