improve --branch processing (and differentiate from # syntax)
Previously #foo and --branch foo were handled identically.
The behavior of #foo hasn't changed, but --branch now works like this:
1) If branchmap is not supported on the remote, the operation fails.
2) If branch is '.', substitute with branch of the working dir parent.
3) If branch exists remotely, its heads are expanded.
4) Otherwise, the operation fails.
Tests have been added for the new cases.