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

Description

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.

This merge request, when approved, will close: It is related to #833 (closed). But does not closes it.

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