Being able to clone sub branches and use of `--with-branches` option

We are now, not obliged to clone the trunk to use the `--with-branches` option!
That way the sub branch will be cloned (with all the branches merged into it)
 and all its children branches will be initialized after.

There is no more technical reasons that prevent cloning a sub branch...

Solve https://groups.google.com/forum/#!topic/git-tfs-dev/XndbEXRcx1M
Merge pull request #703 from pmiossec/clone_subbranch

Being able to clone a sub branches and use of `--with-branches` option