gitweb: Push formatting diff lines to print_diff_chunk()

Now lines are formatted closer to place where we actually use HTML
formatted output.

This means that we put raw lines in the @chunk accumulator, rather than
formatted lines.  Because we still need to know class (type) of line
when accumulating data to post-process and print, process_diff_line()
subroutine was retired and replaced by diff_line_class() used in
git_patchset_body() and new restructured format_diff_line() used in
print_diff_chunk().

As a side effect, we have to pass \%from and \%to down to callstack.

This is a preparation patch for diff refinement highlightning. It's not
meant to change gitweb output.

[jn: wrote commit message]

Signed-off-by: Michał Kiedrowicz <redacted>
Acked-by: Jakub Narębski <redacted>
Signed-off-by: Junio C Hamano <redacted>