gitweb: Add a feature to show side-by-side diff

gitweb: Add a feature to show side-by-side diff

This commits adds to support for showing "side-by-side" style diff.
Currently you have to hand-craft the URL; navigation for selecting
diff style is to be added in the next commit.

The diff output in unified format from "git diff-tree" is reorganized to
side-by-side style chunk by chunk with format_sidebyside_diff_chunk().
This reorganization requires knowledge about diff line classification,
so format_diff_line() was renamed to process_diff_line(), and changed to
return tuple (list) consisting of class of diff line and of
HTML-formatted (but not wrapped in <div class="diff ...">...</div>) diff
line.  Wrapping is now done by caller, i.e. git_patchset_body().

Gitweb uses float+margin CSS-based layout for "side by side" diff.

You can specify style of diff with "ds" ('diff_style') query
parameter.  Currently supported values are 'inline' and 'sidebyside';
the default is 'inline'.

Another solution would be to use "opt" ('extra_options') for that...
though current use of it in gitweb seems to suggest that "opt" is more
about passing extra options to underlying git commands, and "git diff"
doesn't support '--side-by-side' like GNU diff does, (yet?).

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