buildstream/_gitsourcebase.py: Reduce git history for git describe.

Found during #833.

`git rev-list --boundary tag..HEAD` unfortunately gives boundaries
that are deeper than should when there is a merge commit between `tag`
and `HEAD`. The common ancestory of the two parents of the merge is
a boundary instead of the parent of the branch that is not traversed.

`--ancestry-path` fixes this issue by restricting `git` traversing
those branches.