Use --no-commit-id option to git-diff-tree command in git_commit and
git_commitdiff to filter out commit ID output that git-diff-tree adds
when called with only one <tree-ish> (not only for --stdin). Remove
filtering commit IDs from git-diff-tree output.

This option is in git since at least v1.0.0, so make use of it.
Signed-off-by: NJakub Narebski <jnareb@gmail.com>
Signed-off-by: NJunio C Hamano <junkio@cox.net>