gitweb: fix commitdiff for root commits

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>