algos: Add iterators to walk across revisions history
This commit adds iterators to walk across the history of revisions heading
to a given one. The following types of iteration are offered:
- committer_date: revisions are returned in reverse chronological order of their commit date (same as git log)
- dfs: revisions are returned in the same order they are visited when performing a depth-first search in pre order on the revisions DAG
- dfs_post: revisions are returned in the same order they are visited when performing a depth-first search in post order on the revisions DAG
- bfs: revisions are returned in the same order they are visited when performing a breadth-first search on the revisions DAG
Another iterator of type path, returning only revisions that modify a
specific path in reverse chronological order of their commit date, is also
introduced. Nevertheless, due to client-side implementation, its performances
are far from optimal when walking across a really large history.