Fetch merge changesets and checkin merge commits

Now that we have branch support, this is the next step to manage merges with tfs...
This PR permit :
 * to fetch a merge changeset and create a 'mirror' merge commit  (if the tfs merged branch was already inited)
 * to create a merge commit with git and checkin it in tfs as a merge changeset
 * to clone a tfs repository with `--with-branches` option and having git-tfs to init and fetch the trunk and the branches more smartly to fetch merge changesets

this PR could be tested with :
`git tfs clone https://tfs.codeplex.com:443/tfs/TFS16 $/vtccds/trunk . --with-branches --username=vtccds_cp@snd --password=vtccds`