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

Declare commit-graph is per object_source, which may not be a good idea.

* 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

diff --cc commit-graph.c
Simple merge

diff --cc revision.c
Simple merge