gitweb: Extract formatting of diff chunk header

gitweb: Extract formatting of diff chunk header

Refactor main parts of HTML-formatting for diff chunk headers
(formatting means here adding links and syntax hightlighting) into
separate subroutines:

 * format_unidiff_chunk_header for ordinary diff,
 * format_cc_diff_chunk_header for combined diff
   (more than one parent)

This makes format_diff_line() subroutine easier to follow.

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