vcs: introduce 'branches' attribute on changesets, making it possible for Git to show multiple branches for a changeset

Mercurial changesets will always have have exactly one branch (which might be
"default"). The VCS data model was the same.

Git allows for a changeset to have 0 or more branches ... and possibly one of
them as active. The right data model is thus to have an enumerable of branches.

We thus add a 'branches' attribute and use it where applicable.

The existing 'branch' attribute used some heuristics to decide which branch use
as "the" branch ... and in some places code (and tests) rely on that. We thus
keep that old method, knowing that some of its uses probably should move to
'branches'.

The code for retrieving Git branches is based on work by Dominik Ruf.