gitweb: fix commitdiff for root commits
After changing all "-|" open invocations to list form, commitdiff for
initial commit (without parent) got broken; it returned incorrectly
empty patch earlier. Use '--root' option to git-diff-tree for initial
(root) commit.
No checking for empty $hash_parent in git_commitdiff_plain -- we rely
on gitweb to give correct parameters for commitdiff_plain action.
Noticed by Matthias Lederhofer (matled).
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>