Changed behavior of the --branch option (for push, pull etc.)

[Sune Foldager]

I pushed a patch to the list, 3d6915f5a2bb, which changes the behavior 
of the --branch option in push, pull, incoming, outgoing and clone (at 
least). This doesn't implement the full version of what mpm wanted; we 
may adjust the behavior in subsequent commits, if needed.

Developers I talked to at the Swiss mini-sprint found the implemented 
behavior more intuitive, also when you take the documentation of the 
option into account. Other crew members also thought so.

In order to make progress, I pushed this version first, which at least 
is an improvement over the existing code (which treats --branch foo and 
...#foo the same)