add. method _dulwich_tag to Sartoris.  implements tagging from dulwich.

Change-Id: I27b2aaed4418e93ee24c3f59c90f74e9e9467851
add. call to dulwich tag in start.

Change-Id: I182507985dc5d4a87fc135ab02964900d8ad61e4
mod. _create_lock(). File creation should be done with a open write op.

Change-Id: I8d543797d15d6018092ae9e7b458536a2225e456
Merge branch 'gerrit'