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