Enable watching of multiple branches in one repository.

The branches option is now a serie of wildcard matching words such as
'*' (all branches) or 'release*'. All branches found in repo matching these
criteria are followed.

Rather large command language changes. The 'repolog' (former 'log', to
lazy to keep this hazzle) takes a branch parameter. New 'branches' command
lists branches watched in a repo.  Parsing changed to provide possible
values for repo and branch when these are wrong.

Internally, new dict commit_by_branch replaces the old last_commit property.

Left GitPython version 1 compatibility behind, as of now this will break
on the old interface. Possible to update, but is it meaningful?