commit-graph: Introduce `git_commit_list_generation_cmp`

This change makes calculations of merge-bases a bit faster when there
are complex graphs and the commit times cause visiting nodes multiple
times. This is done by visiting the nodes in the graph in reverse
generation order when the generation number is available instead of
commit timestamp. If the generation number is missing in any pair of
commits, it can safely fall back to the old heuristic with no negative
side-effects.

Part of: #5757

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