Previously diff fragments would sometimes expand to the incorrect

place. This is beacuse the generated header information was using

original/patched line numbers instead of the virtual (unified original

and patched) line numbers. Header generation has been modified to use

these line numbers and the issue has been fixed.
Headers also used to sometimes select headers to expand to. The code

that searched for headers wasn't taking into account the difference

between the original/patched line numbers, which are what the headers

use, and the virtual line numbers. This would happen if a deletion or

insertion chunk was above the flagged comment lines.
Update the diff fragment template to allow the left header (which

corresponds to the original file) to be expanded to.
Remove dead code for generation headers from the

get_file_chunks_in_range function. The logic for header generation

wasn't quite right and the only caller of the function is using better

logic to find header information.
Add a unit test to test that the line numbers generated by the new

get_last_header_before_line function are both before the requested

line and have the correct unified line number.

Ran unit tests.
Manually tested the diff fragments with lines flagged that had deletes

and inserts above them. They expanded to the proper locations.
Tested expansion of the left-side of the fragment. It was expandable

and expanded to the correct position.