Fix bug with cross-reference note on commit

It should not set noteable_id if noteable_type is Commit
We have Note#commit_id for this

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
 Merge branch 'bug/noteable_id' into 'master'

Fix bug with cross-reference note on commit

Fixes #1018