gitweb: New improved formatting of chunk header in diff

gitweb: New improved formatting of chunk header in diff

If we have provided enough info, and diff is not combined diff,
and if provided diff line is chunk header, then:
* split chunk header into .chunk_info and .section span elements,
  first containing proper chunk header, second section heading
  (aka. which function), for separate styling: the proper chunk
  header is on non-white background, section heading part uses
  slightly lighter color.
* hyperlink from-file-range to starting line of from-file, if file
  was not created.
* hyperlink to-file-range to starting line of to-file, if file
  was not deleted.
Links are of invisible variety (and "list" class).

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>