Refactore get_commits() to separate function
rename log command -> repolog

Keeping the 'log' commands needs awkward fixes, and the basic problem remains: 'log'
is e very generic name which might pop up in any plugin. Using 'repolog'
instead. refactor checking of repo argument to separate method.
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.

Adding a branch parameter to repolog command.

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?
Adding a 'branches' command