[git-buildpackage] Latest development now always happening on master

[Guido Günther]

Hi,

traditionally gbp used to be developed on the branch that corresponded
to the most current Debian suite i.e. when we uploaded to experimental
(i.e. during a freeze) that would be experimental and when we uploaded
to sid this would be master.

Since gbp is being adopted outside of Debian too I've switched
development to always happen on master, this should make it less
confusing and more like other projects.

So for the future: the latest features are always on *master*, no need
to look at *experimental*.

Cheers,
 -- Guido