gitweb: Added parse_difftree_raw_line function for later use

Adds parse_difftree_raw_line function which parses one line of "raw"
format diff-tree output into a hash.

For later use in git_difftree_body, git_commitdiff and
git_commitdiff_plain, git_search.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>