Merge branch 'ps/commit-graph-per-object-source'

Merge branch 'ps/commit-graph-per-object-source'

Code clean-up around commit-graph.

* ps/commit-graph-per-object-source:
  commit-graph: pass graphs that are to be merged as parameter
  commit-graph: return commit graph from `repo_find_commit_pos_in_graph()`
  commit-graph: return the prepared commit graph from `prepare_commit_graph()`
  revision: drop explicit check for commit graph
  blame: drop explicit check for commit graph