gerritfilterbranch: more factorization

This comes at the prize of a little bit of complexity

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>
gerritchangesource: fix grouping for Nightly schedulers

for nightly and post-commit style schedulers,
you really need to have the branch to be the real gerrit branch
or the list of changes in the UI will be incorrect

This is a simple way of implementing it, probably by separating the
change grouping to the notion of change branch

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>