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.