gitweb: Add parsing of raw combined diff format to parse_difftree_raw_line

gitweb: Add parsing of raw combined diff format to parse_difftree_raw_line

Add parsing line of raw combined diff ("git diff-tree -c/-cc" output)
as described in section "diff format for merges" in diff-format.txt
to parse_difftree_raw_line subroutine.

Returned hash (or hashref) has for combined diff 'nparents' key which
holds number of parents in a merge. At keys 'from_mode' and 'from_id'
there are arrayrefs holding modes and ids, respectively. There is no
'similarity' value, and there is only 'to_file' value and no
'from_file' value.

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